Add extension event histogram values for messaging, webRequest, and webview.

These didn't make it into my last 5 patches because they needed to be hand
written. Now, everything is plumbed through to the UMA_HISTOGRAM_ENUMERATION
calls - which I will add in a follow-up - so this should be the last of it.

BUG=503402
[email protected], [email protected]

Review URL: https://codereview.chromium.org/1272373003

Cr-Commit-Position: refs/heads/master@{#342749}
16 files changed