Actually run WebRTC Win7/Win8 tests on Win7 and Win8.

Bug: 877018
Change-Id: I5258360d3ca5175b6dcc2072c82a1f792f241836
Reviewed-on: https://chromium-review.googlesource.com/1245445Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Patrik Höglund <phoglund@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595395}

infra/config/global/cr-buildbucket.cfg

@@ -3337,11 +3337,31 @@ buckets {
 
     # Win
 
-    builders { mixins: "win"  name: "WebRTC Chromium FYI Win Builder" }
-    builders { mixins: "win"  name: "WebRTC Chromium FYI Win Builder (dbg)" }
-    builders { mixins: "win"  name: "WebRTC Chromium FYI Win10 Tester" }
-    builders { mixins: "win"  name: "WebRTC Chromium FYI Win7 Tester" }
-    builders { mixins: "win"  name: "WebRTC Chromium FYI Win8 Tester" }
+    builders {
+      mixins: "win"
+      name: "WebRTC Chromium FYI Win Builder"
+      dimensions: "os:Windows-10"
+    }
+    builders {
+      mixins: "win"
+      name: "WebRTC Chromium FYI Win Builder (dbg)"
+      dimensions: "os:Windows-10"
+    }
+    builders {
+      mixins: "win"
+      name: "WebRTC Chromium FYI Win10 Tester"
+      dimensions: "os:Windows-10"
+    }
+    builders {
+      mixins: "win"
+      name: "WebRTC Chromium FYI Win7 Tester"
+      dimensions: "os:Windows-7"
+    }
+    builders {
+      mixins: "win"
+      name: "WebRTC Chromium FYI Win8 Tester"
+      dimensions: "os:Windows-8.1"
+    }
   }
 }