Switch Win10 Intel Exp to 2 pools

Switches the win10_intel_hd_630_experimental mixin to trigger tests in
either chromium.tests.gpu.template or
chromium.tests.gpu.experimental.template. The experimental machines will
soon be upgraded to Windows 10 1909 for testing, so we want to move them
to a separae pool so they don't pick up general traffic.

TBR=kbr@chromium.org

Bug: 1053803, 1065505
Change-Id: I9596a030ba089f824654ec610a9bed102641fc1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2154855Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760211}

testing/buildbot/mixins.pyl

@@ -694,7 +694,7 @@
       'dimensions': {
         'gpu': '8086:5912-26.20.100.7870',
         'os': 'Windows-10',
-        'pool': 'chromium.tests.gpu.template',
+        'pool': 'chromium.tests.gpu.template|chromium.tests.gpu.experimental.template',
       },
     },
   },