Make ScrollbarThemeAura selectively invalidate scrollbar parts.

Make ScrollbarThemeAura selectively invalidate scrollbar parts.

In particular:
- when hovering or pressing the thumb, repaint only the thumb
- when thumb position changes, repaint the track only if it changes
  (e.g. due to a button being disabled upon reaching the end)

This CL was broken out of https://codereview.chromium.org/1558493002/
after landing that one lead to issues that are difficult to isolate.

BUG=549277
Committed: https://crrev.com/14716e7723fcfc898c78b52943b53c5a6074194e
Cr-Commit-Position: refs/heads/master@{#370142}

[jbroman, 2016-01-17 00:32:40]

jbroman@chromium.org changed reviewers:
+ skobes@chromium.org

[jbroman, 2016-01-17 00:32:40]

This is the CL that moves Aura over to the piecewise scrollbar invalidation
path, with the unit test from before.

To avoid reintroducing the Mac 10.10 regression on
scheduler.tough_scheduling_cases, I've run the page that reported an issue on:
- Mac 10.10 (looks perf-neutral, as expected)
 
https://build.chromium.org/p/tryserver.chromium.perf/builders/mac_10_10_perf_...
- Linux (looks like an improvement; queueing_durations -4%)
 
https://build.chromium.org/p/tryserver.chromium.perf/builders/linux_perf_bise...

Will be ready to revert if perf bots yell again.

[skobes, 2016-01-19 04:29:51]

lgtm

https://codereview.chromium.org/1591953005/diff/1/third_party/WebKit/Source/p...
File third_party/WebKit/Source/platform/scroll/ScrollableAreaTest.cpp (right):

https://codereview.chromium.org/1591953005/diff/1/third_party/WebKit/Source/p...
third_party/WebKit/Source/platform/scroll/ScrollableAreaTest.cpp:213: 
nit: extra blank line

[jbroman, 2016-01-19 15:08:03]

The CQ bit was checked by jbroman@chromium.org

[jbroman, 2016-01-19 15:08:03]

The patchset sent to the CQ was uploaded after l-g-t-m from skobes@chromium.org
Link to the patchset: https://codereview.chromium.org/1591953005/#ps20001
(title: "merge with master; remove blank line")

[jbroman, 2016-01-19 15:08:06]

https://codereview.chromium.org/1591953005/diff/1/third_party/WebKit/Source/p...
File third_party/WebKit/Source/platform/scroll/ScrollableAreaTest.cpp (right):

https://codereview.chromium.org/1591953005/diff/1/third_party/WebKit/Source/p...
third_party/WebKit/Source/platform/scroll/ScrollableAreaTest.cpp:213: 
On 2016/01/19 at 04:29:51, skobes wrote:
> nit: extra blank line

Done.

[commit-bot: I haz the power, 2016-01-19 15:08:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1591953005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1591953005/20001

[commit-bot: I haz the power, 2016-01-19 17:57:33]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2016-01-19 17:59:00]

Description was changed from

==========
Make ScrollbarThemeAura selectively invalidate scrollbar parts.

In particular:
- when hovering or pressing the thumb, repaint only the thumb
- when thumb position changes, repaint the track only if it changes
  (e.g. due to a button being disabled upon reaching the end)

This CL was broken out of https://codereview.chromium.org/1558493002/
after landing that one lead to issues that are difficult to isolate.

BUG=549277
==========

to

==========
Make ScrollbarThemeAura selectively invalidate scrollbar parts.

In particular:
- when hovering or pressing the thumb, repaint only the thumb
- when thumb position changes, repaint the track only if it changes
  (e.g. due to a button being disabled upon reaching the end)

This CL was broken out of https://codereview.chromium.org/1558493002/
after landing that one lead to issues that are difficult to isolate.

BUG=549277

Committed: https://crrev.com/14716e7723fcfc898c78b52943b53c5a6074194e
Cr-Commit-Position: refs/heads/master@{#370142}
==========

[commit-bot: I haz the power, 2016-01-19 17:59:03]

Patchset 2 (id:??) landed as
https://crrev.com/14716e7723fcfc898c78b52943b53c5a6074194e
Cr-Commit-Position: refs/heads/master@{#370142}