Merge 135316 - When calling DocumentStyleSheetCollection::addUserSheet, pass in a user sheet

Merge 135316 - When calling DocumentStyleSheetCollection::addUserSheet, pass in a user sheet
https://bugs.webkit.org/show_bug.cgi?id=102835

Reviewed by Ojan Vafai.

After r135082, Chromium browser_tests were triggering the ASSERT in
StyleResolver::collectRulesFromUserStyleSheets. Add an ASSERT that will
trigger earlier and make it clear in the Chromium code that we're always
inserting user level styles.

Source/WebCore:

No new tests, no behavior change except no longer triggering the StyleResolver ASSERT
in Chromium browser_tests.

* dom/DocumentStyleSheetCollection.cpp:
(WebCore::DocumentStyleSheetCollection::addUserSheet):

Source/WebKit/chromium:

* src/WebDocument.cpp:
(WebKit::WebDocument::insertUserStyleSheet):


TBR=tony@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=135774