ASSERTION FAILED: extractedStyle in WebCore::ApplyStyleCommand::removeInlineStyleFromElement

The last extractedStyle parameter of removeInlineStyleFromElement() is not mandatory and it's set
to default 0. This way we have to check its existence before the usage.

Backported from my WebKit patch: http://trac.webkit.org/changeset/154439

R=hayato@chromium.org
BUG=350414

Review URL: https://codereview.chromium.org/190993002

git-svn-id: svn://svn.chromium.org/blink/trunk@169948 bbb929c8-8fbe-4397-9dbb-9b2b20218538