Credential Manager: Expose the runtime flag to the embedder.

The credential manager's current implementation exposes sensitive
information to websites via a JavaScript API. Until we have a good feel
for exactly what we want to expose, how we want to expose it, and what
UI needs to be tied to that exposure, we shouldn't enable this via the
general "Experimental Web Features" flag.

We do want to enable testing, though: creating a separate flag on
about:flags seems like a reasonable way of containing the risk.

BUG=400674

Review URL: https://codereview.chromium.org/617003006

git-svn-id: svn://svn.chromium.org/blink/trunk@183065 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[Blink-WebWorkers] WorkerSharedTimer cancels extra delayed tasks.

This CL has the Blink side changes to cancel extra delayed tasks posted
in WorkerSharedTimer. Previously extra tasks were being posted and allowed
to complete if multiple interval timers were scheduled on worker threads.
Now we will cancel any extra delayed task, made redundant due to newly scheduled
lesser timer interval tasks.

BUG=410786

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=182930

Review URL: https://codereview.chromium.org/563203002

git-svn-id: svn://svn.chromium.org/blink/trunk@183066 bbb929c8-8fbe-4397-9dbb-9b2b20218538