Add UMA to measure feasibility of making unique names immutable

r418350 changed Chrome so that unique names are fixed once the frame has
committed the first real load. Unfortunately, the bug still occurs if
there are in-page navigations on the initial empty document mixed with
window.name changes, since the first real load has not yet committed.

One possible simplification is to just make unique name completely
immutable. However, it's possible that some sites depend on creating a
browsing context, naming it, and then navigating it. This measures how
many sites depend on that behavior (though it doesn't measure how many
sites might be broken if this behavior changes).

BUG=607205

Review-Url: https://codereview.chromium.org/2385773003
Cr-Commit-Position: refs/heads/master@{#423411}
3 files changed