DevTools: collect GPU memory usage

Collect used GPU memory and display it on the Counters pane
in timeline memory view.

BUG=326497

Review URL: https://codereview.chromium.org/108223002

git-svn-id: svn://svn.chromium.org/blink/trunk@163557 bbb929c8-8fbe-4397-9dbb-9b2b20218538