[lacros] Use ToT ash-chrome on linux-lacros-rel

This CL starts using ToT ash-chrome on linux-lacros-rel to run tests.

Bug: 1129223
Change-Id: Icc8c51ff0ae6863b999c81d153c5a5905f0961ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487872Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819243}

testing/test.gni

@@ -432,7 +432,13 @@ template("test") {
       ]
 
       if (_use_ash_chrome) {
-        executable_args += [ "--ash-chrome-version=for_bots" ]
+        if (also_build_ash_chrome) {
+          executable_args += [ "--ash-chrome-path=ash_clang_x64/chrome" ]
+        } else {
+          # TODO(crbug.com/1129223): Remove in ~1week after
+          # http://crrev.com/c/2487696 lands and is not reverted.
+          executable_args += [ "--ash-chrome-version=for_bots" ]
+        }
       }
 
       if (is_asan) {