Blink side change to force a Blink heap garbage collection when a GC is forced from V8.

This is used for tests that force garbage collections from V8 to
verify that objects die when expected.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/146693008

git-svn-id: svn://svn.chromium.org/blink/trunk@166134 bbb929c8-8fbe-4397-9dbb-9b2b20218538