Translate the overscroll elasticity layer by the overscroll amount

Translate the overscroll elasticity layer by the overscroll amount

Add a synced property, elastic_overscroll, similar to page_scale_factor.

Use this property to translate the overscroll elasticity layer.

With this in place, the overscroll effect works most of the time,
when using impl-thread events.

Originally committed at r306991, reverted at r306995 due to ASAN break
due to LTI::overscroll_elasticity_layer_ not being initialized.

BUG=133097
TBR=aelias

Committed: https://crrev.com/b9aec4502197bf280b1c66f7104ed78937894b22
Cr-Commit-Position: refs/heads/master@{#307057}

[ccameron, 2014-12-05 18:25:50]

ccameron@chromium.org changed reviewers:
+ aelias@chromium.org

[ccameron, 2014-12-05 18:25:51]

Re-landing this with TBR -- the previous land didn't take because a previous
patch hadn't initialized a variable in LTI (which this patch then tried to check
for NULL).

[ccameron, 2014-12-05 18:25:54]

The CQ bit was checked by ccameron@chromium.org

[commit-bot: I haz the power, 2014-12-05 18:26:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/749393005/1

[commit-bot: I haz the power, 2014-12-05 19:31:12]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-05 19:32:33]

Patchset 1 (id:??) landed as
https://crrev.com/b9aec4502197bf280b1c66f7104ed78937894b22
Cr-Commit-Position: refs/heads/master@{#307057}