Fix bug in Valgrind flags config.

'GPU' is in 'Test-Ubuntu14-GCE-NoGPU-x86_64-Release-Valgrind_CPU' too.

This means we're building it in no-GPU mode, and running it in no-CPU mode.
At least it finishes quite quickly this way (~10 seconds).

BUG=skia:

Review URL: https://codereview.chromium.org/992203004
4 files changed