Update osxbeta target to 10.14.6, also target 10.14.5 for mac 10.14 bot

I also reduced the wait time in the optional dimensions for mac_10.14 bot
from 10 minutes to 5 since I was adding the additional dimension to
(possibly) wait for.

Bug: 968531
Change-Id: Icc7e224c7e7c7fdc72258c9fb898f06d781b4538
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636557Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Lindsay Pasricha <lindsayw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665709}

testing/buildbot/mixins.pyl

@@ -342,20 +342,25 @@
         'os': 'Mac-10.14.4',
       },
       'optional_dimensions': {
-        # Wait 10 minutes for 10.14.4, then fall back to 10.14.3.
+        # Wait 5 minutes for 10.14.3, then 5 for .5, then fall back to 10.14.4
         # The format for optional dimensions is: expiration: [{key, value}, ..].
-        600: [
+        300: [
           {
             'os': 'Mac-10.14.3',
           },
         ],
+        600: [
+          {
+            'os': 'Mac-10.14.5',
+          },
+        ],
       },
     },
   },
   'mac_10.14_beta': {
     'swarming': {
       'dimensions': {
-        'os': 'Mac-10.14.5',
+        'os': 'Mac-10.14.6',
       },
     },
   },