Full color test is the slowest of the unittests, and not catching any
additional bugs at the moment.  Step thru range of 0 to 255 in steps of
5 to speed up the test.  255 is 3 * 5 * 17, so any of those primes would
hit 0 and 255 exactly.

Was LibYUVColorTest.TestFullYUV (896 ms)
Now LibYUVColorTest.TestFullYUV (212 ms)

TBR=kjellander@chromium.org
Bug: libyuv:736
Test: LibYUVColorTest.TestFullYUV
Change-Id: I5b55fb07ada0dc7bdc3c3c20569d36bf09bb3804
Reviewed-on: https://chromium-review.googlesource.com/672064
Commit-Queue: Frank Barchard <fbarchard@google.com>
Reviewed-by: Frank Barchard <fbarchard@google.com>