Add aggregated memory histograms.

Add aggregated memory histograms.

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}