SkTDynamicHash: pick up GetKey(), Hash() from T by default.

SkTDynamicHash: pick up GetKey(), Hash() from T by default.

This also has a somewhat obscure technical benefit: it removes the
requirement that GetKey() and Hash() must be functions with external
linkage, which is required when passing a function pointer to a
template.  A future CL that's run into this problem and the obvious
simplification are about 50/50 why I'm sending this CL.

BUG=skia:
DIFFBASE= https://codereview.chromium.org/222343002/
R=bsalomon@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/222473002

git-svn-id: http://skia.googlecode.com/svn/trunk@14028 2bbb7eff-a529-9590-31e7-b0007b416f81