We now always use custom parsing code for parsing floats in GLSL
shaders. Previously this code was only used in corner cases that
stringstream parsing did not handle according to the GLSL spec.

This is slightly faster in compiler perftests, and results in a
smaller binary as well.

Some new test cases are added to make sure that the custom float
parsing behaves correctly.

BUG=chromium:849245
TEST=angle_unittests, angle_perftests

Change-Id: I2a88ec6a8b427016e34519d72bc98216947a4c64
Reviewed-on: https://chromium-review.googlesource.com/1092697Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>