Have htmlcollection-reachable.html force the one GC needed.

Calling window.gc() slows down test running times, and is
preferably avoided unless needed to verify behavior across
a GC.

For htmlcollection-reachable.html, we only need the
one GC to determine liveness of custom properties for
various platform objects across a garbage collection.

R=
BUG=593888

Review-Url: https://codereview.chromium.org/1934183002
Cr-Commit-Position: refs/heads/master@{#390872}
3 files changed