Revert 251207 "Use gpu::Mailbox instead of std:string in IPCs"

Speculative revert to determine whether it is causing the below
WebViewTest.AutoSizeSW failure on the Linux Test bots (e.g.,
http://build.chromium.org/p/chromium.linux/builders/Linux%20Tests%20%28dbg%29%281%29/builds/30888/steps/browser_tests/logs/AutoSizeSW):

FATAL:texture_layer.cc(141)] Check failed: !mailbox.IsValid() || !holder_ref_
|| !mailbox.Equals(holder_ref_->holder()->mailbox()).

This failure also appeared on the trybot run for this CL.

> Use gpu::Mailbox instead of std:string in IPCs
> 
> - easier to read
> - safer since gpu::Mailbox is constant size and checked in the ParamTraits
> 
> BUG=None
> 
> Review URL: https://codereview.chromium.org/148003006

TBR=piman@chromium.org

Review URL: https://codereview.chromium.org/163433008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251281 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251281