IndexedDB: Simplify WebIDBCallbacks exports, strip IDBFactoryBackend

Eliminate the blink impl of WebIDBCallbacks - implement it in
WebCore (Source/modules). Simultaneously, strip IDBFactoryBackendProxy
down to the bare essentials - an allowIndexedDB() call. (That API
could be moved to Platform or some such in a follow-on patch.)

BUG=234277
R=alecflett@chromium.org,adamk@chromium.org,jamesr@chromium.org

Review URL: https://codereview.chromium.org/67463006

git-svn-id: svn://svn.chromium.org/blink/trunk@162738 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Tune test performance / coverage.

BUG=307721

Review URL: https://codereview.chromium.org/68303003

git-svn-id: svn://svn.chromium.org/blink/trunk@162739 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Optimization in handleStyleSpans() for restricting the for loop to iterate only
over the InsertedNodes (i.e. the inserted fragment).
This is in line with other methods such as removeRedundantStylesAndKeepStyleSpanInline()
and makeInsertedContentRoundTrippableWithHTMLTreeBuilder() that iterate over
the InsertedNodes.

No existing test should fail with this patch.

Review URL: https://codereview.chromium.org/72743002

git-svn-id: svn://svn.chromium.org/blink/trunk@162740 bbb929c8-8fbe-4397-9dbb-9b2b20218538