Oilpan: Force a page navigation GC before starting a V8 GC

This fixes a performance regression of iframe-append-remove.html.

The fact that the PageNavigation GC is scheduled means that there is
a dead frame. In common cases, a sequence of Oilpan's GC => V8 GC =>
Oilpan's GC is needed to collect the dead frame. So we force the
PageNavigation GC before running the V8 GC.

I don't think this CL affects performance of other benchmarks,
since it would be rare to hit a V8 GC between when a PageNavigation GC
gets scheduled and when the event loop finishes (i.e., when the PageNavigation
GC runs).


Review URL: https://codereview.chromium.org/1386453003

Cr-Commit-Position: refs/heads/master@{#352576}
3 files changed