[iOS] Upstream switches and flags controlling which WebView is used

[iOS] Upstream switches and flags controlling which WebView is used

Which of UIWebView and WKWebView to use can be controller by command line
flags, finch and availability (iOS version).

BUG=429756
Committed: https://crrev.com/b53a987a83e2d5ce48de50363cd8202a644d6303
Cr-Commit-Position: refs/heads/master@{#330383}

[sdefresne, 2015-05-18 16:13:26]

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

[sdefresne, 2015-05-18 16:13:27]

Please take a look.

[Eugene But (OOO till 7-30), 2015-05-18 16:47:33]

lgtm

[sdefresne, 2015-05-18 16:50:54]

The CQ bit was checked by sdefresne@chromium.org

[commit-bot: I haz the power, 2015-05-18 16:51:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1144803002/1

[commit-bot: I haz the power, 2015-05-18 20:02:05]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-18 22:40:59]

Patchset 1 (id:??) landed as
https://crrev.com/b53a987a83e2d5ce48de50363cd8202a644d6303
Cr-Commit-Position: refs/heads/master@{#330383}