Make sure to initialize a boolean in ExtensionSettingsHandler

Make sure to initialize a boolean in ExtensionSettingsHandler

It turns out there are 2 constructors, and this boolean was only being
initialized to false in one of them. This caused an extension install
verification request to the verification server on every load of the
chrome://extensions page (regardless of whether the user had any
disabled-due-to-verification-failure extensions).

Also add a histogram for tracking the number of ids that we tried to 
verify which the server replied were invalid (ie not known to be in the
webstore). 


BUG=335379
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=245535

[asargent_no_longer_on_chrome, 2014-01-17 07:44:35]

Finnur - please take a look. If this lg to you, can you also hit the CQ button
for me?

[Finnur, 2014-01-17 12:18:43]

LGTM, in CQ.

[commit-bot: I haz the power, 2014-01-17 12:19:27]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asargent@chromium.org/137893021/20001

[commit-bot: I haz the power, 2014-01-17 14:00:54]

Retried try job too often on win_rel for step(s) browser_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[commit-bot: I haz the power, 2014-01-17 14:51:08]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asargent@chromium.org/137893021/20001

[commit-bot: I haz the power, 2014-01-17 17:00:19]

Change committed as 245535