Make LoadMonitoringExtensionHostQueue remove itself as an ExtensionHost observer at the correct tim…

Make LoadMonitoringExtensionHostQueue remove itself as an ExtensionHost observer at the correct time.

"Extensions.ExtensionHostMonitoring.NumLoaded" UMA is now correctly recorded.

I also cleaned up ExtensionHost and its Start/Stop events a little to make it
more obvious, but in a way that my previous attempt at this (r320536) won't
cause crashes.

R=yoz@chromium.org
TBR=sky@chromium.org

Committed: https://crrev.com/fd474faa8e4f90615bee3b1da22cd4c1d38c5ef2
Cr-Commit-Position: refs/heads/master@{#320806}

[not at google - send to devlin, 2015-03-16 18:43:40]

kalman@chromium.org changed reviewers:
- sky@chromium.org

[not at google - send to devlin, 2015-03-16 18:48:34]

https://codereview.chromium.org/1016473002/diff/1/extensions/browser/extensio...
File extensions/browser/extension_host.cc (right):

https://codereview.chromium.org/1016473002/diff/1/extensions/browser/extensio...
extensions/browser/extension_host.cc:283: if (first_load) {
This, and the has_loaded_once_ check in DidStartLoading, are the only real
differences between this patch and https://codereview.chromium.org/995983002/.
Everything else is renaming "did start/stop load" to "did start/stop *first*
load". Apparently that's what everybody really wants to see.

[Yoyo Zhou, 2015-03-16 20:24:13]

LGTM

[not at google - send to devlin, 2015-03-16 21:06:45]

The CQ bit was checked by kalman@chromium.org

[commit-bot: I haz the power, 2015-03-16 21:07:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1016473002/20001

[commit-bot: I haz the power, 2015-03-16 22:31:11]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-16 22:31:47]

Patchset 2 (id:??) landed as
https://crrev.com/fd474faa8e4f90615bee3b1da22cd4c1d38c5ef2
Cr-Commit-Position: refs/heads/master@{#320806}