Correct mojo::WrapSharedMemoryHandle usage

Fixes some incorrect uses of mojo::WrapSharedMemoryHandle which
were assuming that the call actually has any control over the memory
protection applied to a handle when mapped.

Where fixing usage is infeasible for this CL, TODOs are added to
annotate follow-up work.

Also updates the API and documentation to (hopefully) improve clarity
and avoid similar mistakes from being made in the future.

BUG=792900

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I0578aaa9ca3bfcb01aaf2451315d1ede95458477
Reviewed-on: https://chromium-review.googlesource.com/818282
Reviewed-by: Wei Li <weili@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Yuzhu Shen <yzshen@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530268}
DiscardableSharedMemoryManager::DiscardableSharedMemoryManager()
    : next_client_id_(1),
      default_memory_limit_(GetDefaultMemoryLimit()),
      memory_limit_(default_memory_limit_),
      bytes_allocated_(0),
      memory_pressure_listener_(new base::MemoryPressureListener(
          base::Bind(&DiscardableSharedMemoryManager::OnMemoryPressure,
                     base::Unretained(this)))),
      enforce_memory_policy_task_runner_(base::ThreadTaskRunnerHandle::Get()),
      enforce_memory_policy_pending_(false),
      mojo_thread_message_loop_(nullptr),
      weak_ptr_factory_(this),
      mojo_thread_weak_ptr_factory_(this) {
  DCHECK_NE(memory_limit_, 0u);
  enforce_memory_policy_callback_ =
      base::Bind(&DiscardableSharedMemoryManager::EnforceMemoryPolicy,
                 weak_ptr_factory_.GetWeakPtr());
  base::trace_event::MemoryDumpManager::GetInstance()->RegisterDumpProvider(
      this, "DiscardableSharedMemoryManager",
      base::ThreadTaskRunnerHandle::Get());
  base::MemoryCoordinatorClientRegistry::GetInstance()->Register(this);
}

DiscardableSharedMemoryManager::DiscardableSharedMemoryManager()
    : next_client_id_(1),
      default_memory_limit_(GetDefaultMemoryLimit()),
      memory_limit_(default_memory_limit_),
      bytes_allocated_(0),
      memory_pressure_listener_(new base::MemoryPressureListener(
          base::Bind(&DiscardableSharedMemoryManager::OnMemoryPressure,
                     base::Unretained(this)))),
      enforce_memory_policy_task_runner_(base::ThreadTaskRunnerHandle::Get()),
      enforce_memory_policy_pending_(false),
      mojo_thread_message_loop_(nullptr),
      weak_ptr_factory_(this),
      mojo_thread_weak_ptr_factory_(this) {
  DCHECK_NE(memory_limit_, 0u);
  enforce_memory_policy_callback_ =
      base::Bind(&DiscardableSharedMemoryManager::EnforceMemoryPolicy,
                 weak_ptr_factory_.GetWeakPtr());
  base::trace_event::MemoryDumpManager::GetInstance()->RegisterDumpProvider(
      this, "DiscardableSharedMemoryManager",
      base::ThreadTaskRunnerHandle::Get());
  base::MemoryCoordinatorClientRegistry::GetInstance()->Register(this);
}
