Supporting a playbackTime attribute on AudioprocessingEvent.

According to specification, onaudioprocess() evenhandler should delever playbackTime attribute to JS.
The time when the audio will be played in the same time coordinate system as AudioContext.currentTime.
playbackTime allows for very tight synchronization between processing directly in JavaScript
with the other events in the context's rendering graph.

BUG=332782

Review URL: https://codereview.chromium.org/210973002

git-svn-id: svn://svn.chromium.org/blink/trunk@170505 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert of Revert of Put the squashing layer underneath the ancestor clipping layer if present. (https://codereview.chromium.org/218313002/)

Reason for revert:
The breakage is only with --enable-universal-accelerated-overflow-scroll true. Since that feature is not launched and not tied to squashing, going to re-lad the original CL.

Original issue's description:
> Revert of Put the squashing layer underneath the ancestor clipping layer if present. (https://codereview.chromium.org/215823002/)
> 
> Reason for revert:
> This appears to have unexpected effects on rendering. Bits of gmail are missing for example.
> 
> Original issue's description:
> > Put the squashing layer underneath the ancestor clipping layer if present.
> > 
> > Previously, this was the case for m_graphicsLayer but not m_squashingLayer.
> > Since they now are guaranteed to have the same clipping ancestor, this is both
> > safe and correct.
> > 
> > BUG=336563
> > 
> > Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170348
> 
> TBR=hartmanng@chromium.org,chrishtr@chromium.org
> NOTREECHECKS=true
> NOTRY=true
> BUG=336563
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170413

TBR=hartmanng@chromium.org,vollick@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=336563

Review URL: https://codereview.chromium.org/219223008

git-svn-id: svn://svn.chromium.org/blink/trunk@170507 bbb929c8-8fbe-4397-9dbb-9b2b20218538