Make the header/content separator into a hairline for apps in hidpi

Make the header/content separator into a hairline for apps in hidpi

In order to make the hairline header/content separator look good in HiDPI
- The header/content separator overlays the window controls (to avoid a 1 physical pixel gap between the window controls and the header/content separator in 2x)
- This CL makes the window controls 2 physical pixels taller in 2x
- The bottom 1 physical pixel of the window control images is transparent. This is necessary because the window controls paint on top of the header/content separator

BUG=340135
TEST=Manual, see bug
R=jamescook@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=258301

[pkotwicz, 2014-03-14 19:44:27]

James, can you please take a look?

There was a similar CL that you looked at a while ago that was not quite right.
Unfortunately, getting the hairline requires a whole bunch of rather subtle
things.

[James Cook, 2014-03-14 19:51:47]

I'm just about to head to a school event. I'll look at this Monday, or you can
send it to someone else if you need it sooner.

[James Cook, 2014-03-17 16:55:59]

LGTM

https://codereview.chromium.org/200493004/diff/80001/ash/frame/default_header...
File ash/frame/default_header_painter.cc (right):

https://codereview.chromium.org/200493004/diff/80001/ash/frame/default_header...
ash/frame/default_header_painter.cc:304: // Draw the line as 1px thick
regardless of scale factor.
Thanks for this comment. It saves the reader from having to look up the oddity
of stroke width == 0, which I had to do the last time someone sent me a code
review with this line. :-)

[pkotwicz, 2014-03-20 15:05:22]

Committed patchset #1 manually as r258301 (presubmit successful).