blink/bindings: Report ParkableStrings main thread overhead and savings.

Report the total main thread time overhead of parkable strings to UMA, and the
total savings to memory dumps.
The new histogram is only enabled for foreground string parking, which is the
mode we want to refine and launch as a default.

Also fixes a typo in histograms.xml.

Bug: 924164
Change-Id: I1735e91020b319e47165c4870e01701fefc6602e
Reviewed-on: https://chromium-review.googlesource.com/c/1470229
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632284}
5 files changed