Use double cast to avoid VS 2015 64-bit warning

If you convert a 64-bit pointer directly to a 32-bit or smaller integer
then VS 2015 guesses (not unreasonably) that it might be an accidental
pointer truncation. The standard fix is to use a double-cast, to a
pointer-sized integer and then to DWORD.

This fixes a C4311 "pointer truncation" warning introduced by
crrev.com/1225183003

Warning C4312 (conversion from int to ptr of greater size) is suppressed
in this project but adding double-casts for the reverse conversion keeps
the code symmetrical and allows us to enable C4312 in the future.

BUG=440500

Review URL: https://codereview.chromium.org/1684963003

Cr-Commit-Position: refs/heads/master@{#374828}