DevTools: improve javascript formatter to support "return" properties.

The uglifier tokenizer, which is used under the hood in javascript
formatter, is not clever enough to figure out that the "return" word
in "a.return" is not a "return" statement, but identifier.
This patch works around this issue via verifying the token value after
regex to be a valid identifier.

BUG=218470
R=vsevik, apavlov

Review URL: https://codereview.chromium.org/479033002

git-svn-id: svn://svn.chromium.org/blink/trunk@180467 bbb929c8-8fbe-4397-9dbb-9b2b20218538