IDL compiler: CachedAttribute

Only 3 real examples, all of the form:
[CallWith=ScriptExecutionContext,CachedAttribute=isValueDirty] readonly attribute any value;
...in:
Source/modules/indexeddb/IDBCursor.idl
Source/modules/indexeddb/IDBCursorWithValue.idl

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

BUG=239771

Review URL: https://codereview.chromium.org/25408003

git-svn-id: svn://svn.chromium.org/blink/trunk@158935 bbb929c8-8fbe-4397-9dbb-9b2b20218538