Fix flaky crash in WebPagePopupImpl::closePopup.

Fix flaky crash in WebPagePopupImpl::closePopup.

Reentrance to WebViewImpl::closePagePopup is possible.  closePopup crashed in
that case because m_page was valid and m_page->mainFrame() was nullptr.

The details:
If WebViewImpl::closePagePopup is called, and the LocalFrame in the page popup
has the last references to the page popup owner Element,
 1. WebPagePopupImpl::closePopup() calls destroyPage().
 2. destroyPage() calls Page::willBeDestroyed().
 3. willBeDestroyed() destructs the LocalFrame.
 4. The LocalFrame destructor destructs the owner Element.
 5. The owner Element destructor destructs PickerIndicatorElement.
 6. PickerIndicatorElement destructor calls WebViewImpl::closePagePopup().

This CL changes WebViewImpl::closePagePopup() so that m_pagePopup is cleared
before calling WebPagePopupImpl::closePopup() to avoid reentrancy to it.

BUG=454043
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200528

[tkent, 2015-08-14 05:46:16]

tkent@chromium.org changed reviewers:
+ keishi@chromium.org

[tkent, 2015-08-14 05:46:16]

Keishi, please review this.

[keishi, 2015-08-14 05:51:41]

LGTM

[tkent, 2015-08-14 06:05:43]

The CQ bit was checked by tkent@chromium.org

[commit-bot: I haz the power, 2015-08-14 06:05:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1291303002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1291303002/1

[commit-bot: I haz the power, 2015-08-14 08:32:52]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200528

[tkent, 2015-08-17 02:21:39]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1297883002/ by tkent@chromium.org.

The reason for reverting is: Caused another issue.
.

[tkent, 2015-08-17 02:24:28]

I realized this was not a right fix.
With this patch, PIckerIndicatorElement (implements DateTimeChooserClient) dies
before WebPagePopupImpl notifies the close to PickerIndicatorElement.