Reduce the perf overhead of our dangling markup experiments.

Given that Document::completeURL is a hotspot, this patch revamps the
approach to capture removed whitespace up in //url (where we're already
scanning through the string), and limits the scan for '<' to cases in
which we've actually removed whitespace.

BUG=680970, 682300
R=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2643613002
Cr-Commit-Position: refs/heads/master@{#444734}
8 files changed