cc: Remove empty viewport check in GLRenderer::BeginDrawingFrame.

It's now up to the caller to avoid calls to
::Begin/FinishDrawingFrame when it makes sense to do so.

BUG=428071
R=danakj

Review URL: https://codereview.chromium.org/678113004

Cr-Commit-Position: refs/heads/master@{#302149}

cc/output/gl_renderer.cc

@@ -441,9 +441,6 @@ static ResourceProvider::ResourceId WaitOnResourceSyncPoints(
 }
 
 void GLRenderer::BeginDrawingFrame(DrawingFrame* frame) {
-  if (frame->device_viewport_rect.IsEmpty())
-    return;
-
   TRACE_EVENT0("cc", "GLRenderer::BeginDrawingFrame");
 
   scoped_refptr<ResourceProvider::Fence> read_lock_fence;