The exponent should be rounded prior to casting it to an integer.

Also if the exponent has a significant fractional part the expression
should not be turned into a multiplication. The previous code failed
to check this correctly if the exponent's fractional part was greater
than 0.5.

The test case is expanded to cover the previously failing cases.

BUG=chromium:793115
TEST=angle_end2end_tests

Change-Id: Ic72cd6ddc7f3d2495f7c87a3e3cfac5791445e72
Reviewed-on: https://chromium-review.googlesource.com/817299Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>