Fix additional driver flag on 'KitKat Phone Tester (dbg)'.

Neglected to use --additional-driver-flag command line arg.

Bug: 875172
Change-Id: I3603e0b3659a2e6309673aad66b76a3dcb01fcc0
Reviewed-on: https://chromium-review.googlesource.com/1183884Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584880}

testing/buildbot/chromium.android.json

@@ -3158,7 +3158,7 @@
         "args": [
           "--debug",
           "--android",
-          "--use-gpu-in-tests"
+          "--additional-driver-flag=--use-gpu-in-tests"
         ],
         "experiment_percentage": 100,
         "isolate_name": "webkit_layout_tests_exparchive",

testing/buildbot/test_suite_exceptions.pyl

@@ -1473,7 +1473,7 @@
           '--android',
           # TODO(crbug.com/875172): run on the real GPU since the
           # software fallback, SwiftShader, isn't yet for 32-bit ARM.
-          '--use-gpu-in-tests',
+          '--additional-driver-flag=--use-gpu-in-tests',
         ],
         'experiment_percentage': 100, # crbug.com/824539
       },