Add canvas context type histogram

Add histogram for tracking which names are being used to create canvas
rendering contexts.

BUG=112928

Review URL: https://chromiumcodereview.appspot.com/21861002

git-svn-id: svn://svn.chromium.org/blink/trunk@155620 bbb929c8-8fbe-4397-9dbb-9b2b20218538