Cleanup: Move imageTitle() from LocalizedStrings to ImageDocument.

Our implmenetation of imageTitle() is locale-independent.

BUG=298020
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/24878005

git-svn-id: svn://svn.chromium.org/blink/trunk@158421 bbb929c8-8fbe-4397-9dbb-9b2b20218538