Remove redundant GrContext memory allocation callback code

The memory callback is hooked up to ganesh in content code here:

https://code.google.com/p/chromium/codesearch#chromium/src/content/common/gpu/client/context_provider_command_buffer.cc&l=214

so this code is completely redundant and the only Blink-side use of the
WebGraphicsMemoryAllocation struct.

R=danakj,junov

Review URL: https://codereview.chromium.org/37773005

git-svn-id: svn://svn.chromium.org/blink/trunk@160492 bbb929c8-8fbe-4397-9dbb-9b2b20218538