HashMap misuse in ImageResourceTest.CancelOnDetach unit test

The ImageResourceTest.CancelOnDetach test should not leave its
ImageResource's identifier uninitialized (empty) as this identifier
is used as a key value in the 'ProgressTracker::m_progressItems'
HashMap (passing empty keys to WTF::HashMap leads to undefined
behavior and is not allowed).

Review URL: https://codereview.chromium.org/221933004

git-svn-id: svn://svn.chromium.org/blink/trunk@170749 bbb929c8-8fbe-4397-9dbb-9b2b20218538