Switch browser_tests to Win10 1909.

Capacity was added for these in https://crrev.com/i/3351717.

Bug: 1124829
Change-Id: I8665012f6bad3c2ce8734f51f3e4f6dbf13bbad2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488713Reviewed-by: Ian Struiksma <ianstruiksma@google.com>
Commit-Queue: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819360}

testing/buildbot/chromium.win.json

@@ -1830,7 +1830,7 @@
           "dimension_sets": [
             {
               "cpu": "x86-64",
-              "os": "Windows-10-15063"
+              "os": "Windows-10-18363"
             }
           ],
           "service_account": "chromium-tester@chops-service-accounts.iam.gserviceaccount.com",

testing/buildbot/test_suite_exceptions.pyl

@@ -548,14 +548,6 @@
           '--disable-features=WebRTC-H264WithOpenH264FFmpeg',
         ],
         'swarming': {
-          # TODO(crbug.com/1124829): Remove this exception when moving browser_tests
-          # to 1909.
-          'dimension_sets': [
-            {
-              'cpu': 'x86-64',
-              'os': 'Windows-10-15063',
-            },
-          ],
           # This is for slow test execution that often becomes a critical path of
           # swarming jobs. crbug.com/868114
           'shards': 15,