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Subsequently, remove some static_casts that are no longer needed.

On Windows, while it is possible to create a file mapping of a file >
32-bits long it is not possible to create the view to the file
mapping, so changing this to size_t should have no loss of functionality.

BUG=778316

Change-Id: I81cd2e0808794fdb83fc21c796367369ea2da220
Reviewed-on: https://chromium-review.googlesource.com/755114Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515257}