Use win10 for win-pixel-builder-rel and win-pixel-tester-rel

Because some API is not available for version before win8.1.
https://cs.chromium.org/chromium/src/ui/snapshot/snapshot_win.cc?l=23&rcl=546755634064b30cc88bfca883e61a6af6a0b4ab

Bug: chromium:958242
Change-Id: Ibe738536826ad592f9a687d645a7ddfa69dccbb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692037
Commit-Queue: Sven Zheng <svenzheng@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676776}

testing/buildbot/chromium.fyi.json

@@ -23454,6 +23454,11 @@
         "name": "pixel_browser_tests",
         "swarming": {
           "can_use_on_swarming_builders": true,
+          "dimension_sets": [
+            {
+              "os": "Windows-10-15063"
+            }
+          ],
           "service_account": "chrome-gpu-gold@chops-service-accounts.iam.gserviceaccount.com"
         },
         "test": "pixel_browser_tests"

testing/buildbot/waterfalls.pyl

@@ -1925,6 +1925,13 @@
         }
       },
       'win-pixel-tester-rel': {
+        'swarming': {
+          'dimension_sets': [
+            {
+              'os': 'Windows-10-15063',
+            },
+          ],
+        },
         'test_suites': {
           'gtest_tests': 'pixel_browser_tests_gtests',
         }