IndexedDB: Remove IDBCallbacks base class

No benefit to having IDBCallbacks as a base class for IDBRequest,
and this will assist in further Source/web/*IDB* cleanup.

BUG=234277

Review URL: https://codereview.chromium.org/64553017

git-svn-id: svn://svn.chromium.org/blink/trunk@162264 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Web Animations CSS: Fix DocumentTimeline::numberOfActiveAnimationsForTesting()

This is used by internals.numberOfActiveAnimations() in DRT and is part of the
reason for the failure in repeated-firing-background-color.html

Also beef up animation-controller-drt-api.html to test this fix.

This was originally landed in https://codereview.chromium.org/69813018, but
rolled out due to failures in animations/repeated-firing-background-color.html.
It looks like these were due to timing flakiness, but it doesn't seem worth
investigating further given that this test is only for an internal API and the
fact that the switch to Web Animations is imminent. Added a flaky expectation.

TBR=rjwright,dstockwell
BUG=258896

Review URL: https://codereview.chromium.org/75683004

git-svn-id: svn://svn.chromium.org/blink/trunk@162265 bbb929c8-8fbe-4397-9dbb-9b2b20218538