IDL compiler: Cleanup v8_utilities.py

Misc cleanup (noticed while implementing [DeprecatedAs],
split into separate CL).
* Remove unnecessary 'generate_' prefix
* Return a value instead of modifying |contents|
* Group functions by use
* Consistently use |extended_attributes| local variable
* Sort by Extended Attribute name
* Update docs
* Remove unnecessary |set()|

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/59853004

git-svn-id: svn://svn.chromium.org/blink/trunk@161441 bbb929c8-8fbe-4397-9dbb-9b2b20218538