array.length() should return a signed integer as specified in ESSL 3.00
section 4.1.9, not unsigned. Fix this and add a simple unit test - the
dEQP tests included in WebGL conformance are built in a way that they
don't catch the issue.

TEST=angle_unittests
BUG=angleproject:972

Change-Id: I1389f51751a6a25c1681f57ac3d2d52f31ecc8fb
Reviewed-on: https://chromium-review.googlesource.com/266991Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>