Test WinASan on the same OS version as the main win10 bots

The chromium.win Windows 10 testers use 'Windows-10-15063' as the
swarming os dimension, not 'Windows-10', as I did for the ASan bots.  It
seems that sbox_integration_tests fails on newer versions of Windows 10,
and some other tests might as well. For now, make sure these bots match
the main waterfall. For every test that this fixes, I'll file a
follow-up bug about that test not passing on recent versions of Windows.

TBR=thakis@chromium.org
BUG=844398

Change-Id: I959f2d19459c722e54da9dd09b5fc169e3b171fd
Reviewed-on: https://chromium-review.googlesource.com/1099567Reviewed-by: Reid Kleckner <rnk@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566977}

testing/buildbot/waterfalls.pyl

@@ -536,7 +536,7 @@
         'swarming': {
           'dimension_sets': [
             {
-              'os': 'Windows-10',
+              'os': 'Windows-10-15063',
             },
           ],
         },
@@ -548,7 +548,7 @@
         'swarming': {
           'dimension_sets': [
             {
-              'os': 'Windows-10',
+              'os': 'Windows-10-15063',
             },
           ],
         },
@@ -560,7 +560,7 @@
         'swarming': {
           'dimension_sets': [
             {
-              'os': 'Windows-10',
+              'os': 'Windows-10-15063',
             },
           ],
         },