Split ResourceDispatcherHostImpl::BeginRequest

This method is very long. Factor out code building a resource handler
into a separate method to improve readability.

BUG=none

Review URL: https://codereview.chromium.org/94503003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238346 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238346
Make consistent with Accelerated 2D Canvas flag on both implementation and chrome://gpu.

Currently, we don't enable accelerated 2d canvas in software rendering mode, but
chrome://gpu does not concern software rendering mode.

This CL adds GPUInfo::SupportsAccelerated2dCanvas() so that any clients query if
accelerated 2d canvas is supported.

Review URL: https://codereview.chromium.org/95013004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238347 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238347