Bump tablet tester execution timeout

They seem to need longer timeout for now. Hopefully will be temporary.

TBR=bpastene

Bug: 853248
Change-Id: I6665013085631c69e6d78fe1f43f195e12d3295c
Reviewed-on: https://chromium-review.googlesource.com/1171909Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582420}

infra/config/global/cr-buildbucket.cfg

@@ -142,6 +142,12 @@ builder_mixins {
   }
 }
 
+builder_mixins {
+  # https://crbug.com/853248, should hopefully be temporary
+  name: "android-tablet-ci"
+  execution_timeout_secs: 14400 # 4 hours
+}
+
 builder_mixins {
   name: "android-fyi-ci"
   recipe {
@@ -783,6 +789,7 @@ buckets {
     builders {
       name: "KitKat Tablet Tester"
       mixins: "android-ci"
+      mixins: "android-tablet-ci"
       dimensions: "os:Ubuntu-14.04"
     }
 
@@ -801,6 +808,7 @@ buckets {
     builders {
       name: "Lollipop Tablet Tester"
       mixins: "android-ci"
+      mixins: "android-tablet-ci"
       dimensions: "os:Ubuntu-14.04"
     }
 
@@ -813,6 +821,7 @@ buckets {
     builders {
       name: "Marshmallow Tablet Tester"
       mixins: "android-ci"
+      mixins: "android-tablet-ci"
       dimensions: "os:Ubuntu-14.04"
     }