Chrome forbids RTTI, but standalone ANGLE currently allows it.
Disallowing RTTI in standalone release builds brings ANGLE and Chrome
closer, and prevents accidental use of dynamic_cast in ANGLE.

It also reduces binary size. Win10 libGLESv2 is 47KB smaller after this.

BUG=angleproject:1239

Change-Id: Ib5fb50df66c3a94042e2d9bbb062c0d025cf3eef
Reviewed-on: https://chromium-review.googlesource.com/314831
Tryjob-Request: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Austin Kinross <aukinros@microsoft.com>