Use coverage on the fly for junit tests.

This will turn "coverage on the fly" on if someone uses the
"use_jacoco_coverage" gn arg, which is probably what most people
would expect, however, there wouldn't be a way to turn
--coverage-on-the-fly off with that option.

Bug: 1097815
Change-Id: I07567931854f0d1f6a1399c613bc6f856e1faff7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2298038Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: benjamin joyce <bjoyce@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788476}

build/config/android/internal_rules.gni

@@ -863,6 +863,7 @@ template("test_runner_script") {
         executable_args += [
           "--coverage-dir",
           "@WrappedPath(${_rebased_coverage_dir})",
+          "--coverage-on-the-fly",
         ]
       }
     } else if (_test_type == "linker") {