IDL compiler: implements

The 'implements' statement is already supported, as it's implemented the
same as 'partial interface' (these are both dependenc*y* files).

However, turning on the existing test requires a few auxiliary changes.

This CL:
* adds a test for 'implements' to Python test files
* adds features to Python for new test cases (actually used in real files)
* removes test cases for unused features for which Perl generates incorrect test code

New features:
* EventHandler type method in dependency
* method with arguments in dependency (fix order)
* [RaisesException] on interface type method

Removed test cases:
* [Custom] attribute in dependency
* [PerContextEnabled] attribute in dependency

Also:
* Regularize naming in TestImplements.idl

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/142923002

git-svn-id: svn://svn.chromium.org/blink/trunk@165381 bbb929c8-8fbe-4397-9dbb-9b2b20218538