Distill the concept of the "tab top exclusion region", an area inside the tab

Distill the concept of the "tab top exclusion region", an area inside the tab
bounds which we don't actually want to be part of hit testing or count towards
the gap above tabs in restored mode.

I'd imagine the origin of this is in some kind of change we made to our theme
resources in the past where we made the images taller but didn't actually want
the tab metrics to change, so we just tweaked numbers in various places.

This will have a different size (0) in Material Design, and we'll also use this
value in a future change to fix some alignment problems in theme drawing that
have been introduced over the years.

BUG=none
TEST=none

Committed: https://crrev.com/1a37cd0f8f97ee46836d422c7a659fdb6468ba62
Cr-Commit-Position: refs/heads/master@{#358088}

[Peter Kasting, 2015-11-05 06:58:19]

pkasting@chromium.org changed reviewers:
+ sky@chromium.org

[sky, 2015-11-05 17:02:45]

LGTM

[Peter Kasting, 2015-11-05 17:29:03]

The CQ bit was checked by pkasting@chromium.org

[commit-bot: I haz the power, 2015-11-05 17:29:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1403413006/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1403413006/20001

[commit-bot: I haz the power, 2015-11-05 18:14:25]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-11-05 18:16:03]

Patchset 2 (id:??) landed as
https://crrev.com/1a37cd0f8f97ee46836d422c7a659fdb6468ba62
Cr-Commit-Position: refs/heads/master@{#358088}