IDL parser: Drop Date and RegExp from keywords

Date and RegExp were IDL type, but they were removed in Oct. 2015.[1]
In current spec, they are not reserved words and can be used as
identifiers.

This CL drops these words from keywords in Blink IDL parser, to
follow the current spec.


[1] https://github.com/heycam/webidl/pull/69

Bug: 839389
Change-Id: I2592573ba0c1875a3e828a54adfc5e040184d3fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1696842
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676310}