[clang] Use -Wshorten-64-to-32 to enable warnings about 64bit to 32bit truncations.

Currently only the Win64 bots report this warnings, which adds quite
some overhead to the development process. With this flag we also get
compiler warnings about implicit 64bit to 32bit truncations when
building with clang on Linux/x64 and Mac/x64.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/1111733002

Cr-Commit-Position: refs/heads/master@{#28093}