webview: Initialize synchronous compositor after first activation.

Drawing before the first activation produces no results but can cause
cc DCHECKs to fail. To prevent this SynchronousCompositorImpl doesn't
register with it's client until the first activation.

BUG=492815

Review URL: https://codereview.chromium.org/1150423003

Cr-Commit-Position: refs/heads/master@{#332334}
 void SynchronousCompositorExternalBeginFrameSource::SetCompositor(
     SynchronousCompositorImpl* compositor) {
   DCHECK(CalledOnValidThread());
  if (compositor_ == compositor) return;

  if (compositor_)
    compositor_->OnNeedsBeginFramesChange(false);

   compositor_ = compositor;

  if (compositor_)
    compositor_->OnNeedsBeginFramesChange(needs_begin_frames_);
 }

 void SynchronousCompositorExternalBeginFrameSource::SetCompositor(
     SynchronousCompositorImpl* compositor) {
   DCHECK(CalledOnValidThread());
   compositor_ = compositor;
 }
