Fix broken forward declaration that introduced a clashing namespace

A forward declaration claimed there was a ::viz::cc:CompositorFrame
class, and that introduction of a namespace ::viz::cc caused
confusion with the ::cc namespace and compilation errors in
some jumbo build configurations.

The real CompositorFrame is in viz::CompositorFrame so fixing
the forward declaration also removed the namespace clash.

Change-Id: I1cd4fe729591855172f5a07aca2e2c7f3e8db9fa
Reviewed-on: https://chromium-review.googlesource.com/939390Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#539441}

content/renderer/android/synchronous_compositor_proxy.h

@@ -20,9 +20,7 @@
 #include "ui/gfx/geometry/size_f.h"
 
 namespace viz {
-namespace cc {
 class CompositorFrame;
-}  // namespace cc
 }  // namespace viz
 
 namespace content {