This patch folds --optimize-for-size flag and check for low-memory device
into Heap::ShouldOptimizeForMemoryUsage() predicate.

It has the following side effects:
- the heap growing factor for low-memory devices is capped at 1.3 (old value was 2.0).
- the memory reducer will be more aggressive for low-memory devices.

BUG=chromium:634900

Review-Url: https://codereview.chromium.org/2218703004
Cr-Commit-Position: refs/heads/master@{#38406}