ContentEditable plaintext-only crash fix.

Calling createLink execCommand on a selection that includes both normal
contentEditable and plaintext-only contentEditable caused a crash when
applying style in ApplyStyleCommand::applyInlineStyleToNodeRange because the
EditingStyle::m_mutableStyle is null.

I do not have a well enough understanding of what is going on here to be sure
this is the correct fix, but trying to move the check for style()->style
further up caused various LayoutTests to fail.

BUG=270031

Review URL: https://chromiumcodereview.appspot.com/23600068

git-svn-id: svn://svn.chromium.org/blink/trunk@158248 bbb929c8-8fbe-4397-9dbb-9b2b20218538