The EXT_color_buffer_half_float extension spec issue #2 states that none
of the added color buffer formats are required to be renderable and the
client must check for framebuffer completeness for using them.  Chrome
exposes the extension if at least RGBA16F is renderable for WebGL.

Replicate Chrome's behaviour by loosening our requirements to only
expose this extension if an explicit check for RGBA16F renderability
succeeds.

BUG=angleproject:2984

Change-Id: Id97f3043ebf3fd11b5e9e2d505e57b76baba9716
Reviewed-on: https://chromium-review.googlesource.com/c/1351350
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>