Fix content verifier problem with content scripts (reland)

Fix content verifier problem with content scripts (reland)

The problem is that we were creating the ContentVerifier too late
in ExtensionSystemImpl::Shared::Init, after the script loader had
already tried grabbing a reference to it for use on another
thead. Therefore at run time, for all content scripts we were was
seeing a null pointer for the content verifier and skipping
verification of those scripts.

Note: this is a reland of https://codereview.chromium.org/1226163010/
which was reverted in b9e8b50e4c45d8aefacac58e28c50d83bb257b7b due
to test failures on the Vista Tests bot.

BUG=509130
Committed: https://crrev.com/678123d286ab15d26064122c33f88a9d43673392
Cr-Commit-Position: refs/heads/master@{#341435}

[asargent_no_longer_on_chrome, 2015-07-31 20:15:12]

Patchset #4 (id:60001) has been deleted

[asargent_no_longer_on_chrome, 2015-07-31 20:18:34]

asargent@chromium.org changed reviewers:
+ rockot@chromium.org

[asargent_no_longer_on_chrome, 2015-07-31 20:18:35]

rockot: this is the same CL you reviewed a while ago, with a couple small fixes
to the browser test to eliminate some flakiness that caused it to be reverted.
To see what's changed, look at the diffs between patch set 4 and 1.

[Ken Rockot(use gerrit already), 2015-07-31 22:40:01]

lgtm

[asargent_no_longer_on_chrome, 2015-07-31 22:54:49]

The CQ bit was checked by asargent@chromium.org

[commit-bot: I haz the power, 2015-07-31 22:55:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1250473002/80001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1250473002/80001

[commit-bot: I haz the power, 2015-07-31 23:24:18]

Committed patchset #4 (id:80001)

[commit-bot: I haz the power, 2015-07-31 23:24:50]

Patchset 4 (id:??) landed as
https://crrev.com/678123d286ab15d26064122c33f88a9d43673392
Cr-Commit-Position: refs/heads/master@{#341435}