Avoid stack allocating StyleSheetCollections.

StyleSheetCollection is a GarbageCollected<>-derived object, which ideally
shouldn't be allocated on the stack (== outside the heap), as it leaves
the door open for it being attempted marked (and traced), which it the
allocation doesn't support (the allocation has no header information
preceeding it.)

R=
BUG=

Review-Url: https://codereview.chromium.org/2096653004
Cr-Commit-Position: refs/heads/master@{#413088}
6 files changed