Add presubmit check for run-bindings-tests

run-bindings-tests should be run (and succeed) for any changes to
Source/bindings. This is particularly important to make sure tests are
rebaselined when changes are made to the code generator.
This adds a PRESUBMIT.py script to do just that!

Checked that this worked by changing a test IDL in
  Source/bindings/tests/idls
...and running
  git cl upload
...which then gives a presubmit warning, as desired!

Script based on GetUnitTests in
tools/depot_tools/presubmit_canned_checks.py
...which seems exemplary. I've stripped it down to the minimum. Ref:
http://src.chromium.org/viewvc/chrome/trunk/tools/depot_tools/presubmit_canned_checks.py?view=markup

Proximately, this is in reply to Erik's request:
https://codereview.chromium.org/19047003/#msg5

Review URL: https://chromiumcodereview.appspot.com/21722003

git-svn-id: svn://svn.chromium.org/blink/trunk@155448 bbb929c8-8fbe-4397-9dbb-9b2b20218538