Make Clang's update.py use the Win toolchain under depot_tools if available

Clang should be built with the same toolchain used to build Chromium,
which is typically the one in depot_tools/win_toolchain.

BUG=82385
[email protected]

Review URL: https://codereview.chromium.org/587383003

Cr-Commit-Position: refs/heads/master@{#296090}
2 files changed