sk_fgetsize to not use ftell.

The previous version of sk_fgetsize used ftell and fseek to compute
the size of a file. There are so many issues with this that it is called
out by securecoding.cert.org as FIO19-C as a thing not to do. We already
have correct code for computing the size of a file in the mmap code, so
use that instead.

Change-Id: I1d771124989d0ec1523f6d858814ee563263213a
Reviewed-on: https://skia-review.googlesource.com/9860
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
3 files changed