Rename usesOverlayScrollbars of settings to mockScrollbarsUseOverlay

Rename usesOverlayScrollbars of settings to mockScrollbarsUseOverlay

usesOverlayScrollbars takes effect only if mockScrollbarsEnabled.
Rename it to mockScrollbarsUseOverlay to avoid misuse.

The existing misuse is that some tests call only
window.internals.setUsesOverlayScrollbars(true) that will do nothing
without window.interals.settings.setMockScrollbarsEnabled(true).

Besides renaming, removed the original wrong placed
window.internals.setUsesOverlayScrollbars() and added
window.internals.settings.setMockScrollbarsUseOverlay().

Removed ScrollbarThemeMock::usesOverlayScrollbars() because
ScrollbarThemeMock is not used when overlayScrollbars is enabled
(ScroolbarThemeOverlayMock is used).

Updated the tests that origially call setUsesOverlayScrollbar(true)
only.

BUG=none
TEST=existing layout tests

[Xianzhu, 2013-09-12 22:54:42]

ping...

[Xianzhu, 2013-09-13 23:09:53]

On 2013/09/12 22:54:42, Xianzhu wrote:
> ping...

ping...

[ojan, 2013-09-13 23:45:12]

lgtm

[commit-bot: I haz the power, 2013-09-13 23:45:46]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/wangxianzhu@chromium.org/23819053/7001

[commit-bot: I haz the power, 2013-09-13 23:45:54]

Failed to apply patch for LayoutTests/fast/dom/partial-layout-block.html:
While running patch -p1 --forward --force --no-backup-if-mismatch;
  patching file LayoutTests/fast/dom/partial-layout-block.html
  Hunk #1 FAILED at 27.
  1 out of 1 hunk FAILED -- saving rejects to file
LayoutTests/fast/dom/partial-layout-block.html.rej

Patch:       LayoutTests/fast/dom/partial-layout-block.html
Index: LayoutTests/fast/dom/partial-layout-block.html
diff --git a/LayoutTests/fast/dom/partial-layout-block.html
b/LayoutTests/fast/dom/partial-layout-block.html
index
df8d31d3efeb7bbc5bb20b7e948ba7bc34a03129..8973a6e5f234150b9cdb175753f55ab895cdc933
100644
--- a/LayoutTests/fast/dom/partial-layout-block.html
+++ b/LayoutTests/fast/dom/partial-layout-block.html
@@ -27,8 +27,10 @@ html, body {
     if (window.testRunner)
         testRunner.dumpAsText();
 
-    if (window.internals)
-        window.internals.setUsesOverlayScrollbars(true);
+    if (window.internals) {
+        internals.settings.setMockScrollbarsEnabled(true);
+        internals.settings.setMockScrollbarsUseOverlay(true);
+    }
 
     test(function() {
         var measure = document.getElementById('measure');

[Xianzhu, 2013-09-13 23:54:06]

Thanks ojan. I'll rebase and cq soon.

[Xianzhu, 2013-09-14 00:30:48]

+tony who landed a similar change.

[Xianzhu, 2013-09-14 00:33:55]

Closing. Seems unnecessary after Tony's change. Thanks Tony for fixing the
issue.