Add SiteIsolation.IsolateExtensions metrics and tests.
These new UMA metrics estimate the process-count impact of a policy
wherein extension content and web content never share processes,
even if one contains the other in an iframe.
SiteDetailsBrowserTest: add tests for the new metrics.
WebContentsImpl: Fix a UMA metrics counting error where
<iframe src="about:blank"> was counted as a distinct site.
Now we'll only do that if we see "about:blank" in an
undifferentiated SiteInstance.
BUG=528049
Review URL: https://codereview.chromium.org/
1313863006
Cr-Commit-Position: refs/heads/master@{#348690}