Support "shared" histograms between processes.

Histograms have tradionally been held on the local heap and
passed between processes using RPCs or simply lost when that
process stops.  Shared histograms are read directly by another
process.
https://docs.google.com/document/d/1YvBXzi745UDjiFVLjP8IWUMFgYL9gDb2J4xuwcv-c7o/edit?usp=sharing

BUG=546019
TBR=mmenke,nasko,thakis
for base/*.{gn,gypi}, components/cronet/*, and content/child/*

Review URL: https://codereview.chromium.org/1425533011

Cr-Commit-Position: refs/heads/master@{#372760}
24 files changed