add a Win/Clang build bot

BUG=skia:2679

CQ_INCLUDE_TRYBOTS=skia.primary:Build-Win-Clang-x86_64-Release

Change-Id: I861366ae1340abcc67e87041904d08337b465bbe
Reviewed-on: https://skia-review.googlesource.com/28864
Reviewed-by: Eric Boren <borenet@google.com>
7 files changed