Split out content/child's SiteIsolationPolicy into two new classes.

New classes are CrossSiteDocumentClassifier and SiteIsolationStatsGatherer.
The latter employs the former. In a follow-on change I'll split these
into two files, and move CrossSiteDocumentClassifier into content/common.
SiteIsolationStatsGatherer will stay where it is.

BUG=481066, 268640

Review URL: https://codereview.chromium.org/1181493002

Cr-Commit-Position: refs/heads/master@{#334755}
12 files changed