GL_CHROMIUM_color_buffer_float_rgb[a] allows these sized formats to be
used in TexImage2D even in ES2.  With this patch, the
conformance/extensions/oes-texture-float and
conformance/extensions/oes-texture-half-float tests now pass for WebGL1
and WebGL 2 contexts.

BUG=angleproject:1958

Change-Id: I568dea5da42ba310463d2690c3e764c48598311b
Reviewed-on: https://chromium-review.googlesource.com/522349Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>