Add chromium.memory swarming dimensions

Bug: 880973
Change-Id: Ic92339a48740e4e18781d81861252c9fc6424d2a
Reviewed-on: https://chromium-review.googlesource.com/1239629
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593704}

testing/buildbot/waterfalls.pyl

@@ -3179,6 +3179,10 @@
         ],
       },
       'Linux ChromiumOS MSan Tests': {
+        'swarming_mixins': [
+          'linux-trusty',
+          'x86-64',
+        ],
         'test_suites': {
           'gtest_tests': 'linux_chromeos_gtests',
         },
@@ -3187,6 +3191,10 @@
         ],
       },
       'Linux MSan Tests': {
+        'swarming_mixins': [
+          'linux-trusty',
+          'x86-64',
+        ],
         'test_suites': {
           'gtest_tests': 'chromium_linux_and_gl_gtests',
         },