Validate aom_tune_metric value.
Validate aom_tune_metric value.

Change-Id: I6d0f42ddec857921f5beb52b25925747e6412956

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom