This introduces V8.MemoryHeapCommitted and V8.MemoryHeapUsed histograms.

In contrast to the existing memory histograms, the new histograms are uniform in time, i.e. their samples happen at regular time intervals. The --histogram-interval specifies the length of the interval.

We implement this by linearly interpolating memory stats between GC and idle notification events.

BUG=chromium:485472
LOG=NO

Review URL: https://codereview.chromium.org/1125683004

Cr-Commit-Position: refs/heads/master@{#28292}