Fix use-after-free in WebstoreInstaller

Fix use-after-free in WebstoreInstaller

Crashes are still occurring in WebstoreInstaller::StartDownload(),
even after several attempts to fix them.

Diagnosis:
The installer holds a pointer to contents::NavigationController.
This object is owned by contents::WebContents, which can be deleted
before the StartDownload() callback is invoked.

Fix:
Store a pointer to contents::WebContents. Inherit from 
content::WebContentsObserver, which will clear the pointer when
the WebContents are destroyed.

BUG=165634
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=250865

[tmdiep, 2014-02-10 23:56:44]

asargent: I saw the comments about the crashes while I was in this code and had
a hunch. benjhayden confirms that the crashes still occur and Opera is reporting
them too. I'm submitting this patch to see whether the crash incidence
decreases.

Is this a reasonable diagnosis? Thanks.

[tmdiep, 2014-02-11 22:32:58]

Opera team members came to the same conclusion and submitted an essentially
identical fix:
https://codereview.chromium.org/145873003

[asargent_no_longer_on_chrome, 2014-02-11 22:41:12]

lgtm

[tmdiep, 2014-02-12 21:58:47]

The CQ bit was checked by tmdiep@chromium.org

[commit-bot: I haz the power, 2014-02-12 22:02:27]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/tmdiep@chromium.org/138803012/1

[commit-bot: I haz the power, 2014-02-13 00:26:09]

Change committed as 250865