Re-add snap-back behavior (Chrome side)

After Aura copied the original Linux scrollbar behvaior, it never got updated
with Windows-specific native behaviors from ScrollbarThemeWin.cpp:

* Snap the scrollbar back to its drag origin when dragged too far off the track.
This code matches the old code we had for years and also matches my Win 7
native scrollbars pixel-for-pixel.

* Don't jump the scroll thumb to the mouse cursor on a middle-click, only on a
shift-click. Middle-click must be a GTK-ism.

This is the Chrome side of the changes to re-add this behavior.

BUG=337919
TEST=Drag a scroll thumb to somewhere in the track, then without releasing, move the mouse far away from the track. The scroll thumb should snap back to the scroll origin.
R=avi@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=254305

[Peter Kasting, 2014-02-24 23:57:58]

Avi, can you review this?  The Blink side is at
https://codereview.chromium.org/151483008/ for reference.

You implemented the previous plumbing of the "jump on track click" Mac scroll
behavior preference, so I especially want to know if the way it's plumbed here
makes sense, or if there is some sort of further simplification I can do to
avoid the remaining existing plumbing of that pref down to the renderer.

[Avi (use Gerrit), 2014-02-25 03:44:31]

lgtm

https://codereview.chromium.org/175903002/diff/80001/content/content_renderer...
File content/content_renderer.gypi (right):

https://codereview.chromium.org/175903002/diff/80001/content/content_renderer...
content/content_renderer.gypi:563:
'renderer/webscrollbarbehavior_impl_gtkoraura.h',
Huh. Do we do this anywhere else? Kind of a bummer that we have to do this for
GTK which is going away soon.

[Peter Kasting, 2014-02-25 06:10:02]

https://codereview.chromium.org/175903002/diff/80001/content/content_renderer...
File content/content_renderer.gypi (right):

https://codereview.chromium.org/175903002/diff/80001/content/content_renderer...
content/content_renderer.gypi:563:
'renderer/webscrollbarbehavior_impl_gtkoraura.h',
On 2014/02/25 03:44:31, Avi wrote:
> Huh. Do we do this anywhere else? Kind of a bummer that we have to do this for
> GTK which is going away soon.

What specific thing are you referring to when you say "this"?

I could put everything in one file with ifdefs, but separating the Mac
implementation out seemed cleaner since it was different enough from the other
two.  I don't really have a strong feeling for how I should have split the code,
named the filed, or structured the .gypi changes, though; I was mostly aiming
for something that worked, and I was somewhat inspired by the "Mac" vs.
"GTKorAura" split in the ScrollbarTheme code over in Blink.

[Peter Kasting, 2014-03-01 01:06:32]

Committed patchset #7 manually as r254305 (presubmit successful).