Otherwise on Windows or other platforms with GLES backend, using this workaround
will trigger shader compile failure.

For the moment, to simplify the workaround, for any missing precisions in float,
we always assign the highest available precion to it.

BUG=angle:695
TEST=webgl conformance tests

Change-Id: I810579442bc7fc61d6a52b76aff31a779046fa22
Reviewed-on: https://chromium-review.googlesource.com/211253Tested-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>