Removed pinch viewport scroll offset distribution

The associated change in Blink makes the pinch viewport a proper
ScrollableArea meaning the normal path for synchronizing layer scroll
offsets is used.

This is a 2 sided patch, the other CL:
https://codereview.chromium.org/199253002/

BUG=349941

Review URL: https://codereview.chromium.org/210543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260105 0039d316-1c4b-4281-b951-d872f2087c98
void LayerTreeHost::SetNeedsCommit() {
  if (!prepaint_callback_.IsCancelled()) {
    TRACE_EVENT_INSTANT0("cc",
                         "LayerTreeHost::SetNeedsCommit::cancel prepaint",
                         TRACE_EVENT_SCOPE_THREAD);
    prepaint_callback_.Cancel();
  }
  proxy_->SetNeedsCommit();
  NotifySwapPromiseMonitorsOfSetNeedsCommit();
}

void LayerTreeHost::SetNeedsCommit() {
  if (!prepaint_callback_.IsCancelled()) {
    TRACE_EVENT_INSTANT0("cc",
                         "LayerTreeHost::SetNeedsCommit::cancel prepaint",
                         TRACE_EVENT_SCOPE_THREAD);
    prepaint_callback_.Cancel();
  }
  proxy_->SetNeedsCommit();
  NotifySwapPromiseMonitorsOfSetNeedsCommit();
}
