Make GrScratchKey memory buffer correct size on copy

Make GrScratchKey memory buffer correct size on copy

Scratch key memory buffer of a copy of a key was too big. The (new) copy
was N times uint32_t bytes instead of N bytes.

Adds few tests to resource cache. These tests would not catch the too
big buffer. This is just a precaution for too small buffers. The main
idea of the test change is that the scratch key should contain some
information, so that lookup with a scratch key can also return no
match. Otherwise testing of scratch lookup result is not indicative of
correct code (eg. no-information scratch key will always match).

Review URL: https://codereview.chromium.org/860333002