Merge 94102 - [Chromium] Add null checks for document()->loader()

Merge 94102 - [Chromium] Add null checks for document()->loader()
https://bugs.webkit.org/show_bug.cgi?id=67077

Reviewed by Nate Chapin.

Document::loader can return 0 for a number of reasons.  Call sites need
to null check the result, like we do for Document::frame.  I suspect
some of these are real crashes, along the lines of
http://trac.webkit.org/changeset/93692, but null checking this function
is just good practice.

* src/ContextMenuClientImpl.cpp:
(WebKit::ContextMenuClientImpl::getCustomMenuFromDefaultItems):
* src/WebSearchableFormData.cpp:
(HTMLNames::GetFormEncoding):
* src/WebViewImpl.cpp:
(WebKit::WebViewImpl::pageEncoding):


TBR=abarth@webkit.org
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=94713