DevTools: [Elements] Fix tag name editing after committing an empty last attribute

Appending an attribute at the last position killed the tree element DOM and
did not restore it if an empty value was committed.

R=aandrey, lushnikov, vsevik
BUG=391645

Review URL: https://codereview.chromium.org/378843002

git-svn-id: svn://svn.chromium.org/blink/trunk@177690 bbb929c8-8fbe-4397-9dbb-9b2b20218538