Add the Windows.IsPinnedToTaskbar metric.

This metric records the taskbar pinned state of Chrome for the current
user. Because the code loads shell extensions into the process, this
is done on a utility process.

BUG=621662

Review-Url: https://codereview.chromium.org/2079233004
Cr-Commit-Position: refs/heads/master@{#403695}
19 files changed