Implement IDBVersionChangeEvent ctor w/ IDBVersionChangeEventInit dict

A rarely used part of the IndexedDB spec was overlooked - like most
other events, script can call the constructor with a dict to populate
the fields.

This required some finessing: the fields were stored as 'IDBAny'
but our automagical code gen wiring for EventInit prefers ScriptValue
or SerializedScriptValue. I went with the latter, and as an aside
eliminate a call to v8::Isolate::GetCurrent() by SSV - serializing
simple values such as null or numbers can be done with no isolate.

BUG=345881
R=dgrogan,haraken

Review URL: https://codereview.chromium.org/177633006

git-svn-id: svn://svn.chromium.org/blink/trunk@168162 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Enable round-tripping and updating of SourceBuffer timestamp offset

This is the second part of a three-sided MediaSource 
Chromium/Blink+roll Blink/Chromium set of changes to enable 
JavaScript web apps to continue to round-trip 
SourceBuffer.timestampOffset set/get if SourceBuffer append 
operations do not update the timestamp offset, and to 
retrieve any updated timestamp offset as can occur during 
upcoming "sequence" append mode compliant coded frame 
processing. 

R=acolwell@chromium.org,jamesr@chromium.org
BUG=249422
TEST=No local linux regressions of http/tests/media/media-source/ layout tests

Review URL: https://codereview.chromium.org/178763006

git-svn-id: svn://svn.chromium.org/blink/trunk@168163 bbb929c8-8fbe-4397-9dbb-9b2b20218538