Copy and paste can strip !important CSS rules due to a bug in mergeStyleFromRules

Copy and paste can strip !important CSS rules due to a bug in mergeStyleFromRules
https://bugs.webkit.org/show_bug.cgi?id=115217

Reviewed by Darin Adler.

Source/WebCore:

The bug was caused by mergeStyleFromRules overriding "important" style rules with "unimportant" inline styles.
Fixed the bug by using addParsedProperty, which respects !important, in MutableStylePropertySet's
mergeAndOverrideOnConflict, which was only used in editing code. Now that we've fixed this function, we can use
it in ViewportStyleResolver::addViewportRule as well.

Test: editing/pasteboard/copy-paste-with-important-rules.html

* css/StylePropertySet.cpp:
(WebCore::MutableStylePropertySet::mergeAndOverrideOnConflict): Fixed to respect !important.
* css/ViewportStyleResolver.cpp:
(WebCore::ViewportStyleResolver::addViewportRule): Use mergeAndOverrideOnConflict now that the code is identical.

LayoutTests:

Added a regression test.

* editing/pasteboard/copy-paste-with-important-rules-expected.txt: Added.
* editing/pasteboard/copy-paste-with-important-rules.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@149167 268f45cc-cd09-0410-ab3c-d52691b4dbfc