Fix GpuLayerCacheTest compile error

Fix GpuLayerCacheTest compile error:
GpuLayerCacheTest.cpp:120:34: error: comparison between signed and
unsigned integer expressions [-Werror=sign-compare]

gcc --version
gcc (Ubuntu/Linaro 4.6.4-1ubuntu1~12.04) 4.6.4

Review URL: https://codereview.chromium.org/794003003
Fix float and half float support on mobile.

It's unclear what params should be used for half float alpha
rendertargets, so they are disabled for the moment.

Review URL: https://codereview.chromium.org/799593002