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}
size_t RenderProcessHost::GetMaxRendererProcessCount() {
  if (g_max_renderer_count_override)
    return g_max_renderer_count_override;

#if defined(OS_ANDROID)
  static const size_t kNumRendererSlots =
      ChildProcessLauncher::GetNumberOfRendererSlots();
  return kNumRendererSlots;
#endif
#if defined(OS_CHROMEOS)
  return 100;
#endif


  static size_t max_count = 0;
  if (!max_count) {
    const size_t kEstimatedWebContentsMemoryUsage =
#if defined(ARCH_CPU_64_BITS)
        60;  // In MB
#else
        40;  // In MB
#endif
    max_count = base::SysInfo::AmountOfPhysicalMemoryMB() / 2;
    max_count /= kEstimatedWebContentsMemoryUsage;

    const size_t kMinRendererProcessCount = 3;
    max_count = std::max(max_count, kMinRendererProcessCount);
    max_count = std::min(max_count, kMaxRendererProcessCount);
  }
  return max_count;
}

size_t RenderProcessHost::GetMaxRendererProcessCount() {
  if (g_max_renderer_count_override)
    return g_max_renderer_count_override;

#if defined(OS_ANDROID)
  static const size_t kNumRendererSlots =
      ChildProcessLauncher::GetNumberOfRendererSlots();
  return kNumRendererSlots;
#endif
#if defined(OS_CHROMEOS)
  return 100;
#endif


  static size_t max_count = 0;
  if (!max_count) {
    const size_t kEstimatedWebContentsMemoryUsage =
#if defined(ARCH_CPU_64_BITS)
        60;  // In MB
#else
        40;  // In MB
#endif
    max_count = base::SysInfo::AmountOfPhysicalMemoryMB() / 2;
    max_count /= kEstimatedWebContentsMemoryUsage;

    const size_t kMinRendererProcessCount = 3;
    max_count = std::max(max_count, kMinRendererProcessCount);
    max_count = std::min(max_count, kMaxRendererProcessCount);
  }
  return max_count;
}
