Oilpan: avoid heap allocation in MajorGCWrapperVisitor

The collection of retained object roots is preferably not allocated on
the Oilpan heap as that risks triggering an unnecessary GC.

R=haraken
BUG=

Review URL: https://codereview.chromium.org/1539133002

Cr-Commit-Position: refs/heads/master@{#367161}
1 file changed