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.

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}
4 files changed