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Create a separate RenderWidgetResizeHelper to handle intercepting 
and early-processing of IPCs that need to be run during an active 
resize. This functionality was previously handled in 
RenderWidgetHelper. 

The only functional changes made to RenderWidgetResizeHelper during 
its excision is that it is a single global structure, as compared with
the per-RenderProcessImpl RenderWidgetHelper. It will execute any IPCs
from any renderer or the GPU processes that are forwarded to it. This is
preferable, as that makes it so that live resize in one window does 
not destroy performance in other windows. 

Because RenderWidgetResizeHelper can handle GPU process IPCs, we can 
tear out the bizarre behavior where we'd translate GPU IPCs to synthetic 
renderer IPCs get them to channel through RenderWidgetHelper (this is 
all of the deleted ViewHostMsg_CompositorSurfaceBuffersSwapped code). 

Mark this entire structure as Mac-only, and put it in Mac-only files,
since it doesn't get used on other platforms.

Future changes will make RenderWidgetResizeHelper have a TaskRunner, 
which we will pass to the compositor, so that we can pump new frames 
while inside -[RenderWidgetHostViewMac setFrameSize:].

BUG=392031

Review URL: https://codereview.chromium.org/396483003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283726 0039d316-1c4b-4281-b951-d872f2087c98