Fix OverscrollWindowDelegate to not dispatch superfluous OverscrollModeChange events

Fix OverscrollWindowDelegate to not dispatch superfluous OverscrollModeChange events

Previously OverscrollWindowDelegate sometimes would call OnOverscrollModeChange on its delegate even if the overscroll mode didn't change. Specifically this happened for OVERSCROLL_NONE when an overscroll-cancelling condition such as a mouse move would occur.

This could confuse OverscrollWindowAnimation and make it cancel the overscroll animation which shouldn't have been cancelled. On Windows there is currently a mouse event dispatched after each swipe on the touch screen, so this caused crbug.com/493672.

BUG=493672
Committed: https://crrev.com/404f4d1ddaa75235e67a5f8a71ecc6ac8b2d65ec
Cr-Commit-Position: refs/heads/master@{#333049}

[mfomitchev, 2015-06-04 23:10:50]

mfomitchev@chromium.org changed reviewers:
+ sadrul@chromium.org

[mfomitchev, 2015-06-04 23:11:27]

Sadrul, can you PTAL? This is needed for M44.

[sadrul, 2015-06-05 14:26:52]

lgtm

[mfomitchev, 2015-06-05 14:27:44]

The CQ bit was checked by mfomitchev@chromium.org

[commit-bot: I haz the power, 2015-06-05 14:28:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1167013002/20001

[commit-bot: I haz the power, 2015-06-05 15:36:15]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-05 15:37:13]

Patchset 2 (id:??) landed as
https://crrev.com/404f4d1ddaa75235e67a5f8a71ecc6ac8b2d65ec
Cr-Commit-Position: refs/heads/master@{#333049}