Make ListHashSet allocation lazy.
Make ListHashSet allocation lazy.

ListHashSet has custom allocation logic that begins
by greedily allocating a pool that can hold entries.
The custom allocator was added more than 6 years ago
with no test. This showed up in profiles since we
have a ListHashSet in RenderLineBoxList::paint, but
it's frequently unused.

Locally, I see from a 1-17% speedup running the
Blink Layout perf tests.

BUG=313294

Review URL: https://codereview.chromium.org/60253003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@162777 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs