[code coverage] Set a default coverage output directory for junit tests

Currently only set default output directory for instrumentation tests https://cs.chromium.org/chromium/src/build/config/android/internal_rules.gni?type=cs&q=%22--coverage-dir%22&sq=package:chromium&g=0&l=763-772
Should do the same for junit tests as well.
Will use this default directory to collect coverage data on bot.


Bug: 970021
Change-Id: I346a8a1493622e61634eebd340b4a8c1c64e93e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1641379Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Commit-Queue: Yun Liu <yliuyliu@google.com>
Cr-Commit-Position: refs/heads/master@{#665949}

build/config/android/internal_rules.gni

@@ -792,6 +792,16 @@ template("test_runner_script") {
         "--robolectric-runtime-deps-dir",
         "@WrappedPath(${_rebased_robolectric_runtime_deps_dir})",
       ]
+      if (jacoco_coverage) {
+        # Set a default coverage output directory (can be overridden by user
+        # passing the same flag).
+        _rebased_coverage_dir =
+            rebase_path("$root_out_dir/coverage", root_build_dir)
+        executable_args += [
+          "--coverage-dir",
+          "@WrappedPath(${_rebased_coverage_dir})",
+        ]
+      }
     } else if (_test_type == "linker") {
       executable_args += [
         "--test-apk",