[iOS] ios/web no longer depends on content::BrowserThread.

[iOS] ios/web no longer depends on content::BrowserThread.

Remove the shim implementation of web::WebThread based on
content::BrowserThread now that Chrome on iOS uses ios/web
for the startup.

Add call to WebThreadImpl::FlushThreadPoolHelperForTesting()
to WebTestSuite to mirror ContentTestSuite.

BUG=547725
Committed: https://crrev.com/d646dbcd24bb3219407803357c9239938232a8fc
Cr-Commit-Position: refs/heads/master@{#357330}

[sdefresne, 2015-10-27 17:52:01]

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

[sdefresne, 2015-10-27 17:52:01]

Please take a look.

stuartmorgan: FYI

[droger, 2015-10-27 18:03:30]

lgtm

[sdefresne, 2015-10-28 15:30:59]

Patchset #2 (id:20001) has been deleted

[sdefresne, 2015-11-02 10:32:43]

The CQ bit was checked by sdefresne@chromium.org

[sdefresne, 2015-11-02 10:32:43]

The patchset sent to the CQ was uploaded after l-g-t-m from droger@chromium.org
Link to the patchset: https://codereview.chromium.org/1406983008/#ps80001
(title: "Rebase")

[commit-bot: I haz the power, 2015-11-02 10:32:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1406983008/80001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1406983008/80001

[commit-bot: I haz the power, 2015-11-02 10:59:32]

Committed patchset #4 (id:80001)

[commit-bot: I haz the power, 2015-11-02 11:00:37]

Patchset 4 (id:??) landed as
https://crrev.com/d646dbcd24bb3219407803357c9239938232a8fc
Cr-Commit-Position: refs/heads/master@{#357330}