Always create an instance when testing GrEffects for SkXfermode and SkArithmeticMode

Previously the TestCreate methods of the effects would create a static
instance and return that for all invocations. This is probably a
copy-paste error going back to initial TestCreate method in r8449. At
that time, the effect was always the same and thus static usage made
sense.

R=bsalomon@google.com, senorblanco@google.com

Author: kkinnunen@nvidia.com

Review URL: https://codereview.chromium.org/39393002

git-svn-id: http://skia.googlecode.com/svn/trunk@11939 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed