Set deceleration rate for WKWebView

Set deceleration rate for WKWebView

Matches the current setting for UIWebView. Note that this won't have
any effect in older version of iOS since it was only recently fixed
by http://trac.webkit.org/changeset/188541

BUG=520995
Committed: https://crrev.com/0792f679c5b3e9acbbf05650da294029a20f8aa7
Cr-Commit-Position: refs/heads/master@{#352081}

[stuartmorgan, 2015-10-02 17:38:56]

stuartmorgan@chromium.org changed reviewers:
+ eugenebut@chromium.org

[Eugene But (OOO till 7-30), 2015-10-02 17:41:37]

lgtm

[stuartmorgan, 2015-10-02 17:58:58]

The CQ bit was checked by stuartmorgan@chromium.org

[commit-bot: I haz the power, 2015-10-02 18:00:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1386693002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1386693002/20001

[commit-bot: I haz the power, 2015-10-02 18:16:10]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-02 18:17:11]

Patchset 2 (id:??) landed as
https://crrev.com/0792f679c5b3e9acbbf05650da294029a20f8aa7
Cr-Commit-Position: refs/heads/master@{#352081}