Fix an NPE in BrowserPluginGuest during guest teardown while showing interstitial.

Fix an NPE in BrowserPluginGuest during guest teardown while showing interstitial.

r324002 introduced a code path where we call
BrowserPluginGuest::SendTextInputTypeChangedToView() during shutdown via
BrowserPluginGuest::SetFocus(). This happens when guest is showing
interstitial page.

There are couple of ways this can happen, either when the embedder page
closes or the embedder page navigates to a different page.
GuestViewBase::OwnerContentsObserver detects these two cases and calls
BrowserPluginGuest shutdown path. During that time if guest was showing an
interstitial, then it closes the interstitial which in that case would call:

content::InterstitialPageImpl::DontProceed() /
content::InterstitialPageImpl::Hide()
Hide() ends up calling BrowserPluginGuest::SetFocus() and we hit the NPE.

BUG=477126, 477132
Test=Navigate a tab chrome signin page, after changing the system
clock to few years back, observe that it is shows interstitial.

Either of the following reproduces the behavior:
a) close the tab
b) navigate the tab to some webpage, e.g. http://test.com

Committed: https://crrev.com/8664beb06cdff77b4f02154d47bd1f762d2b895a
Cr-Commit-Position: refs/heads/master@{#325193}

[lazyboy, 2015-04-15 01:55:41]

lazyboy@chromium.org changed reviewers:
+ fsamuel@chromium.org

[Fady Samuel, 2015-04-15 02:11:10]

lgtm

[lazyboy, 2015-04-15 03:48:06]

The CQ bit was checked by lazyboy@chromium.org

[commit-bot: I haz the power, 2015-04-15 03:48:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1089123002/1

[commit-bot: I haz the power, 2015-04-15 06:12:47]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-15 06:14:38]

Patchset 1 (id:??) landed as
https://crrev.com/8664beb06cdff77b4f02154d47bd1f762d2b895a
Cr-Commit-Position: refs/heads/master@{#325193}