Make --enable-webgl-draft-extensions about:flags non-expiring.

This flag is the only way to test upcoming WebGL extensions on
Android, and it must not be removed.

Bug: None
Change-Id: I75fbacea512c5a48e25f922d59a82143a2944512
Reviewed-on: https://chromium-review.googlesource.com/c/1476235


Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633426}