Crash in Web Content Process under ~PDFDocument under clearTouchEventListeners at...

Crash in Web Content Process under ~PDFDocument under clearTouchEventListeners at topDocument()
https://bugs.webkit.org/show_bug.cgi?id=135319
<rdar://problem/17315168>

Reviewed by Darin Adler and Antti Koivisto.

* WebProcess/WebCoreSupport/WebFrameLoaderClient.cpp:
(WebKit::WebFrameLoaderClient::committedLoad):
Allow data through to WebCore for frames with custom content providers;
the only custom content provider currently implemented is main frame PDF
on iOS, which will end up creating a PDFDocument in WebCore, which drops all
data on the floor immediately, so this won't result in WebCore doing anything
with the data, but makes sure that more of the normal document lifecycle is maintained.

In the future, we might want to consider ensuring that all custom content providers
end up creating a SinkDocument or something similarly generic to ensure that
WebCore doesn't try to do anything with their data, but for now, the only client is covered.

* dom/Document.h:
* dom/Document.cpp:
(WebCore::Document::Document):
(WebCore::Document::prepareForDestruction):
Add a flag on Document, m_hasPreparedForDestruction, which ensures
that each Document only goes through prepareForDestruction() once.
prepareForDestruction() can be called a number of times during teardown,
but it's only necessary to actually execute it once.

This was previously achieved by virtue of all callers of prepareForDestruction()
first checking hasLivingRenderTree, and prepareForDestruction() tearing down
the render tree, but that meant that prepareForDestruction() was not called
for Documents who never had a render tree in the first place.

The only part of prepareForDestruction() that is now predicated on hasLivingRenderTree()
is the call to destroyRenderTree(); the rest of the function has the potential to be relevant
for non-rendered placeholder documents and can safely deal with them in other ways.

It is important to call prepareForDestruction() on non-rendered placeholder documents
because some of the cleanup (like disconnectFromFrame()) is critical to safe destruction.

* loader/FrameLoader.cpp:
(WebCore::FrameLoader::clear):
Call prepareForDestruction() even if we don't have a living render tree.
For the sake of minimizing change, removeFocusedNodeOfSubtree still
depends on having a living render tree before calling prepareForDestruction().

* page/Frame.cpp:
(WebCore::Frame::setView):
(WebCore::Frame::setDocument):
Call prepareForDestruction() even if we don't have a living render tree.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@171647 268f45cc-cd09-0410-ab3c-d52691b4dbfc