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
Extract a minimal set of classes for observing the lifecycle of a context.

From ContextLifecycleObserver extract LifecycleObserver. From ContextLifecycleNotifier extract
LifecycleNotifier. From ScriptExecutionContext extract LifecycleContext.

Use these minimal classes to observe Page. In this case it is used to observe page visibility in the
Vibration API. When the page is no longer visible all running vibrations must be cancelled.

BUG=222504
R=abarth@chromium.org, loislo@chromium.org

Review URL: https://codereview.chromium.org/18777003

git-svn-id: svn://svn.chromium.org/blink/trunk@154401 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add missing files after r154401

BUG=none
TBR=mvanouwerkerk@chromium.org

Review URL: https://codereview.chromium.org/19574005

git-svn-id: svn://svn.chromium.org/blink/trunk@154402 bbb929c8-8fbe-4397-9dbb-9b2b20218538