Run clang win x64 tests on Win10 swarming machines

This aligns the chromium.clang waterfall swarming configuration with the
configuration used for chromium.win.

Use the 'win10' mixin as a shorter spelling for:
'swarming': { 'dimension_sets': { 'os': 'Windows-10-1503' } } }
... and use that on the CrWinASan bots, which are also all x64 bots.

R=hans@chromium.org, thakis@chromium.org
NOTRY=true

Bug: 939003
Change-Id: I52d33f47b37c0973336695269a499ca9ad2373c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1505894
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638733}

testing/buildbot/waterfalls.pyl

@@ -707,28 +707,22 @@
         },
       },
       'CrWinAsan': {
+        'mixins': [
+          'x86-64',
+          'win10',
+        ],
         'test_suites': {
           'gtest_tests': 'chromium_win_gtests',
         },
-        'swarming': {
-          'dimension_sets': [
-            {
-              'os': 'Windows-10-15063',
-            },
-          ],
-        },
       },
       'CrWinAsan(dll)': {
+        'mixins': [
+          'x86-64',
+          'win10',
+        ],
         'test_suites': {
           'gtest_tests': 'chromium_win_gtests',
         },
-        'swarming': {
-          'dimension_sets': [
-            {
-              'os': 'Windows-10-15063',
-            },
-          ],
-        },
       },
       'ToTAndroid': {
         'additional_compile_targets': [
@@ -884,16 +878,28 @@
         },
       },
       'ToTWin64': {
+        'mixins': [
+          'x86-64',
+          'win10',
+        ],
         'test_suites': {
           'gtest_tests': 'chromium_win_gtests',
         },
       },
       'ToTWin64(dbg)': {
+        'mixins': [
+          'x86-64',
+          'win10',
+        ],
         'test_suites': {
           'gtest_tests': 'chromium_win_gtests',
         },
       },
       'ToTWin64(dll)': {
+        'mixins': [
+          'x86-64',
+          'win10',
+        ],
         'test_suites': {
           'gtest_tests': 'chromium_win_gtests',
         },