coverage: Move the CLANG_COVERAGE define in better place.

Instead of defining CLANG_COVERAGE for everything, define it
only where this is actually used.

BUG=none
TBR=avi

Change-Id: I5e09df66cb44cdd6598af8f0d474aa9b7a309460
Reviewed-on: https://chromium-review.googlesource.com/c/1425961
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624783}
9 files changed