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NPEvent::lParam is defined as uintptr_t which has same width with ULONG on Win32, but they are of different sizes on WIN64. uintptr_t is 8-byte long, while ULONG is still 4-byte long, so it corrupts a 64-bit pointer if casting it to a 32-bit type.

BUG=413535
TEST=square.html (attached in the bug), http://pbskids.org/video (it uses wmode="opaque")
R=ananta@chromium.org, bauerb@chromium.org

Review URL: https://codereview.chromium.org/571583002

Cr-Commit-Position: refs/heads/master@{#294917}