Remove duplicate storage of fCanvas in SkOverdrawCanvas

BUG=skia:

Change-Id: I06827e286b13823fdd89090a75fc63467c9a74e9
Reviewed-on: https://skia-review.googlesource.com/5294
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
2 files changed