Stop sending low memory notifications to V8

Now that we (should) report the size of external memory to V8, the GC
heuristic of V8 should be good enough.

BUG=260596
R=abarth@chromium.org,haraken@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19350003

git-svn-id: svn://svn.chromium.org/blink/trunk@154400 bbb929c8-8fbe-4397-9dbb-9b2b20218538