Remove Platform::requestProcessMemorySizes

Minor refactoring. I couldn't find its usage anywhere.
It is not implemented nor called.

BUG=None

Review URL: https://codereview.chromium.org/378503002

git-svn-id: svn://svn.chromium.org/blink/trunk@177596 bbb929c8-8fbe-4397-9dbb-9b2b20218538