fix bench so that if SK_SUPPORTS_GPU even when not provided as compiler option

fix bench so that if SK_SUPPORTS_GPU even when not provided as compiler option

In the case that SK_SUPPORTS_GPU is not provided to the compiler the value is
eithe defined in in the SkUserConfig.h or SkPostConfig.h.  Prior to this change
those headers were not read prior to trying to include the GPU headers which
resulted in a failed compile.

R=bsalomon@google.com

Review URL: https://codereview.chromium.org/76213004

git-svn-id: http://skia.googlecode.com/svn/trunk@12309 2bbb7eff-a529-9590-31e7-b0007b416f81