Revert of Disable kEnableGpuMemoryBufferCompositorResources on Mac (patchset...

Revert of Disable kEnableGpuMemoryBufferCompositorResources on Mac (patchset #1 id:1 of https://codereview.chromium.org/1435133003/ )

Reason for revert:
This is no longer needed now that 48 has branched and https://codereview.chromium.org/1463853002/ is landed.

Original issue's description:
> Disable kEnableGpuMemoryBufferCompositorResources on Mac
>
> This caused an uptick in crashes due to GpuMemoryBuffers failing to
> allocate.
>
> BUG=554541
> TBR=reveman
>
> Committed: https://crrev.com/c5b6a85810bd669a3916587ac4d70ac566351258
> Cr-Commit-Position: refs/heads/master@{#359396}

TBR=
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=554541

Review URL: https://codereview.chromium.org/1470213002

Cr-Commit-Position: refs/heads/master@{#361248}

content/browser/gpu/compositor_util.cc

@@ -240,9 +240,7 @@ bool IsGpuMemoryBufferCompositorResourcesEnabled() {
     return false;
 
 #if defined(OS_MACOSX)
-  // Disabled due to increased crash rates.
-  // http://crbug.com/554541
-  return false;
+  return true;
 #else
   return false;
 #endif