IDL compiler: Remove capitalization special cases from Python, add tests

Followup to:
IDL compiler: remove special case capitalization (cssText, xmllang, xmlbase, xmlspace)
https://codereview.chromium.org/26896005/
...removes from Python CG, and rather simplifies the capitalization function.
Also adds test cases for acronyms actually used with setters.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/27903003

git-svn-id: svn://svn.chromium.org/blink/trunk@159943 bbb929c8-8fbe-4397-9dbb-9b2b20218538
IDL compiler: [CachedAttribute] for setters

One-line change to generated setters.

Docs:
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-CachedAttribute-a-

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/28163002

git-svn-id: svn://svn.chromium.org/blink/trunk@159944 bbb929c8-8fbe-4397-9dbb-9b2b20218538