cc: Simplify raster task completion notification logic

(Relanding after missing activation bug fixed in https://codereview.chromium.org/131763003/)

Previously the pixel buffer raster worker pool used a combination of
polling and explicit notifications from the raster worker pool to decide
when to tell the client about the completion of 1) all tasks or 2) the
subset of tasks required for activation. This patch simplifies the logic
by only triggering the notification based on the OnRasterTasksFinished
and OnRasterTasksRequiredForActivationFinished calls from the worker
pool.

BUG=307841,331534

Review URL: https://codereview.chromium.org/99873007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243991 0039d316-1c4b-4281-b951-d872f2087c98
unsigned PixelBufferRasterWorkerPool::PendingRasterTaskCount() const {
  unsigned num_completed_raster_tasks =
      tasks_with_pending_upload_.size() + completed_tasks_.size();
  DCHECK_GE(pixel_buffer_tasks_.size(), num_completed_raster_tasks);
  return pixel_buffer_tasks_.size() - num_completed_raster_tasks;
}

unsigned PixelBufferRasterWorkerPool::PendingRasterTaskCount() const {
  unsigned num_completed_raster_tasks =
      tasks_with_pending_upload_.size() + completed_tasks_.size();
  DCHECK_GE(pixel_buffer_tasks_.size(), num_completed_raster_tasks);
  return pixel_buffer_tasks_.size() - num_completed_raster_tasks;
}
