Bug 1558598 - Add the --enable-webrender option to android HW unittests. r=ahal

This is not used yet but will be eventually so I'm just going to
add it now.

As of this patch, all the tests that we currently run on android HW do
accept the --enable-webrender flag and explicitly disable WR if it is
not provided.

Differential Revision: https://phabricator.services.mozilla.com/D35868

--HG--
extra : moz-landing-system : lando

testing/mozharness/scripts/android_hardware_unittest.py

@@ -77,6 +77,13 @@ class AndroidHardwareTest(TestingMixin, BaseScript, MozbaseMixin,
          "default": True,
          "help": "Run tests without multiple processes (e10s).",
          }
+    ], [
+        ['--enable-webrender'],
+        {"action": "store_true",
+         "dest": "enable_webrender",
+         "default": False,
+         "help": "Run with WebRender enabled.",
+         }
     ]] + copy.deepcopy(testing_config_options)
 
     def __init__(self, require_config_file=False):
@@ -118,6 +125,7 @@ class AndroidHardwareTest(TestingMixin, BaseScript, MozbaseMixin,
         self.log_raw_level = c.get('log_raw_level')
         self.log_tbpl_level = c.get('log_tbpl_level')
         self.e10s = c.get('e10s')
+        self.enable_webrender = c.get('enable_webrender')
 
     def query_abs_dirs(self):
         if self.abs_dirs:
@@ -247,6 +255,14 @@ class AndroidHardwareTest(TestingMixin, BaseScript, MozbaseMixin,
             elif category not in SUITE_DEFAULT_E10S and self.e10s:
                 cmd.extend(['--e10s'])
 
+        # Only enable WebRender if the flag is enabled. All downstream harnesses
+        # are expected to force-disable WebRender if not explicitly enabled,
+        # so that we don't have it accidentally getting enabled because the
+        # underlying hardware running the test becomes part of the WR-qualified
+        # set.
+        if self.enable_webrender:
+            cmd.extend(['--enable-webrender'])
+
         try_options, try_tests = self.try_args(self.test_suite)
         cmd.extend(try_options)
         if not self.verify_enabled and not self.per_test_coverage: