Framebuffer clear() needs to consider the situation some draw buffers are disabled.

This is when we expose DrawBuffers extension.

BUG=376951
TEST=the attached test case, webgl conformance
R=kbr@chromium.org,bajones@chromium.org

Review URL: https://codereview.chromium.org/315283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275338 0039d316-1c4b-4281-b951-d872f2087c98
BackRenderbuffer::BackRenderbuffer(
    RenderbufferManager* renderbuffer_manager,
    MemoryTracker* memory_tracker,
    ContextState* state)
    : renderbuffer_manager_(renderbuffer_manager),
      memory_tracker_(memory_tracker, MemoryTracker::kUnmanaged),
      state_(state),
      bytes_allocated_(0),
      id_(0) {
}

BackRenderbuffer::BackRenderbuffer(
    RenderbufferManager* renderbuffer_manager,
    MemoryTracker* memory_tracker,
    ContextState* state)
    : renderbuffer_manager_(renderbuffer_manager),
      memory_tracker_(memory_tracker, MemoryTracker::kUnmanaged),
      state_(state),
      bytes_allocated_(0),
      id_(0) {
}
