Merge: Allow offsets in blobs larger than 2GB on 32 bit Chromium builds.

Merge: Allow offsets in blobs larger than 2GB on 32 bit Chromium builds.

The recent patch crrev.com/895933007 introduced a regression by using size_t
for offsets, which on 32bit builds is sizeof(size_t) = 4 which was causing
broken offset values due to assigning a uint64_t variable to such variables.

This CL fixes that by simply converting the types to uint64_t.

TBR=michaeln@chromium.org  # For merge.
TEST=*FileSystemProvider*BigFile* on a 32 bit build Chromium build with
    chromeos=1.
BUG=375297, 458122
Review URL: https://codereview.chromium.org/959113002

Cr-Commit-Position: refs/heads/master@{#318808}
(cherry picked from commit f6629637feb3bbf186de75da8fe4028a5244389f)

Committed: https://chromium.googlesource.com/chromium/src/+/35feed79c8c72b9ab539f7750a35d4c6001c01b7

[mtomasz, 2015-03-03 01:45:41]

Committed patchset #1 (id:1) manually as
35feed79c8c72b9ab539f7750a35d4c6001c01b7 (presubmit successful).