IDL compiler: cleanup [ConstructorTemplate] => [EventConstructor]

Cleanup, no changes to generated code.

Only [ConstructorTemplate=Event] is used in actual IDLs;
this renames it to clearer [EventConstructor],
and deletes code and test case for unused [ConstructorTemplate=TypedDef].

Also a bit of renaming and cleanup of test cases.

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

Review URL: https://codereview.chromium.org/107753002

git-svn-id: svn://svn.chromium.org/blink/trunk@163337 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Generate toRenderTextFragment with DEFINE_TYPE_CASTS, and apply it to remaining static_cast<>.

BUG=309516

Review URL: https://codereview.chromium.org/107823002

git-svn-id: svn://svn.chromium.org/blink/trunk@163338 bbb929c8-8fbe-4397-9dbb-9b2b20218538