During editing, merge inline style with overriding other author styles

Inline styles need to override other author styles even on DoNotOverrideValues mode.
So, merge and override inline styles to other author styles before merging them to m_mutableStyle.

Test: editing/deleting/merge-div-with-inline-style.html

Merge from WebKit: http://webkit.org/b/122874

BUG=371312

Review URL: https://codereview.chromium.org/272753003

git-svn-id: svn://svn.chromium.org/blink/trunk@178739 bbb929c8-8fbe-4397-9dbb-9b2b20218538