[SVG] Static tearoffs should mark contextElement when instantiated.

Tearoffs must mark their contextElement so that they are never marked as partially-dependent. The animated tearoffs have done this, but static tearoffs didn't. This patch fixes the issue.

BUG=320762

Review URL: https://codereview.chromium.org/59753010

git-svn-id: svn://svn.chromium.org/blink/trunk@162274 bbb929c8-8fbe-4397-9dbb-9b2b20218538
IDL compiler: Remove extended attributes from typedefs

Simplification; no changes to generated code of real IDL files.

Extended attributes are no longer allowed on typedefs,
per latest ED.
We don't use them in any real IDL files (only 1 test), and they
significantly complicate the parser.
This CL removes them, and thus can also remove the IdlTypedef class;
typedefs are now just a simple string -> string dict.

Editor's Draft (latest):
http://heycam.github.io/webidl/#idl-typedefs

Candidate Recommendation (old):
http://www.w3.org/TR/WebIDL/#idl-typedefs

BUG=239771

Review URL: https://codereview.chromium.org/67353005

git-svn-id: svn://svn.chromium.org/blink/trunk@162275 bbb929c8-8fbe-4397-9dbb-9b2b20218538