[code coverage] Moving merge scripts src-side.

This is the first part of moving the code coverage merge scripts to src.
A recipe-side change will follow once this has landed.

R=martiniss,liaoyuke,sajjadm
BUG=928577

Change-Id: I213f13651a4a6440ef3fbd9e813ff7371de32ec9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1521465
Commit-Queue: Roberto Carrillo <robertocn@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640613}
5 files changed