win10: Fix various ui glitches

This addresses the most egregious problems so that we look more
or less like a reasonable app on Win10.

In particular, fixes a too-wide border, removing the rounded corners
on the toolbar that don't work when there's no border (see 504044 for
agreement from UX), and an ugly overhang in the non-client area in
the top-left and top-right of the window.

In case you're not familiar with Win10 UI style (I wasn't!) I put
a composite of what standard apps plus before/after this patch here
http://i.imgur.com/JxNJvBv.png . We aren't easily able to switch to
a more "grey"-style title bar at the moment as that's reserved for
Windows-store applications (plain Win32 apps like notepad and chrome
always get the white title bar style). We might want to look into
that in the future as it's pretty harsh, but at least we match the
expected system behaviour this way.

R=pkasting@chromium.org
BUG=504044,502001

Review URL: https://codereview.chromium.org/1209723004

Cr-Commit-Position: refs/heads/master@{#336438}

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