Change GC ergonomics to better handle heap verification.

Added a new constant kMaxConcurrentRemainingBytes which put a lower
bound on the background GC watermark. This fixes a performance issue
caused by heap verification being really slow and making it that we
were excessively scheduling back to back GCs.

Greatly increases system/test performance when heap verification is
enabled.

Change-Id: I68fa5a247c42f6c661e313c49dd3ca6be29951d4