chromium_tests recipe module currently checks if builder_cache base path
is defined. If it is not, it let's the caller to decide where to put
the cache. All the callers choose start_dir.
https://cs.chromium.org/search/?q=get_checkout_dir&type=cs

In LUCI we want to use [CACHE]/builder as builder cache. To do that we
want to update chromium_tests recipe module to use [CACHE]/builder if
builder_cache is not defined.

Define builder_cache in buildbot to point to start dir, so that in LUCI
it is not defined and thus [CACHE]/builder is used.

R=iannucci@chromium.org
BUG=660481

Change-Id: I9018da5c43a5396add6a26ad2c12b1755fbe6307
Reviewed-on: https://chromium-review.googlesource.com/457439Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>