Fix executable bit on aggregate_generated_bindings.py

Executable bit is wrong (not executable),
but file actually is executable, and has a shebang,
hence make executable.

This causes problems when trying to edit the file,
as presubmit scripts fail.

TBR=haraken

Review URL: https://codereview.chromium.org/192363004

git-svn-id: svn://svn.chromium.org/blink/trunk@168813 bbb929c8-8fbe-4397-9dbb-9b2b20218538