Revert "sk_fgetsize to not use ftell."

Revert "sk_fgetsize to not use ftell."

This reverts commit eefa289a214ea3917e5e00554f231adb5ad6f712.

Reason for revert: Causing failures in BitmapFactoryTest. b/38233042

Original change's description:
> 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>
>

TBR=bungeman@google.com,scroggo@google.com
# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Ie9392dde8747ae7c74ebfa00153705e316e841a2
Reviewed-on: https://skia-review.googlesource.com/17705
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>