Revert of make getContentKey() available in GrGpuResource public interface (patchset...

Revert of make getContentKey() available in GrGpuResource public interface (patchset #2 id:20001 of https://codereview.chromium.org/886313005/)

Reason for revert:
This causes in one case the linker and in the other the compiler to segfault. On

Test-Ubuntu12-ShuttleA-GTX660-x86_64-Debug/Release

Original issue's description:
> make getContentKey() available in GrGpuResource public interface
>
> TBR=robertphillips@google.com
>
> Committed: https://skia.googlesource.com/skia/+/85ed2db092e75db41b711b9116a8d5b465fc2b0c

TBR=robertphillips@google.com,bsalomon@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/864343005