Fix a 64-bit pointer truncation bug found by VC++ 2015
Fix a 64-bit pointer truncation bug found by VC++ 2015

When converting from void* to unsigned long long it is dangerous to go
through unsigned long because for VC++ 64-bit builds this will be 32
bits. When casting a pointer to an integral type the safest type to
choose for the integral cast is always intptr_t or uintptr_t.

BUG=440500
NOPRESUBMIT=true

Review URL: https://codereview.webrtc.org/1437433002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10569}

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