Migrate manually triggered GPU trybots to Xenial

TBR=kbr

Bug: 981736
Change-Id: Idc05ad4da8e07742e9110b1a8c2c74d10f6f6e97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1707568Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678477}

infra/config/cr-buildbucket.cfg

@@ -357,13 +357,21 @@ builder_mixins {
 }
 
 builder_mixins {
-  name: "android-optional-gpu-try"
+  name: "android-optional-gpu-try-trusty"
   dimensions: "os:Ubuntu-14.04"
   mixins: "android-try"
   mixins: "gpu-optional-try"
   mixins: "gpu-slow-bot"
 }
 
+builder_mixins {
+  name: "android-optional-gpu-try"
+  dimensions: "os:Ubuntu-16.04"
+  mixins: "android-try"
+  mixins: "gpu-optional-try"
+  mixins: "gpu-slow-bot"
+}
+
 builder_mixins {
   name: "angle-try"
   service_account: "chromium-try-gpu-builder@chops-service-accounts.iam.gserviceaccount.com"
@@ -609,12 +617,27 @@ builder_mixins {
 }
 
 builder_mixins {
-  name: "linux-optional-gpu-try"
+  name: "linux-optional-gpu-try-trusty"
   mixins: "linux-try"
   mixins: "gpu-optional-try"
   mixins: "gpu-slow-bot"
 }
 
+builder_mixins {
+  name: "linux-optional-gpu-try"
+  mixins: "linux-xenial-try"
+  mixins: "gpu-optional-try"
+  mixins: "gpu-slow-bot"
+}
+
+builder_mixins {
+  name: "linux-xenial-try"
+  mixins: "linux-xenial"
+  recipe {
+    properties: "mastername:tryserver.chromium.linux"
+  }
+}
+
 builder_mixins {
   name: "linux-try"
   mixins: "linux"
@@ -3753,7 +3776,7 @@ buckets {
       mixins: "linux-xenial"
       mixins: "builderless"
     }
-    builders { mixins: "android-optional-gpu-try" name: "android_optional_gpu_tests_rel" }
+    builders { mixins: "android-optional-gpu-try-trusty" name: "android_optional_gpu_tests_rel" }
     builders {
       mixins: "android-try"
       mixins: "goma-rbe-prod"
@@ -4254,7 +4277,7 @@ buckets {
       mixins: "linux-xenial"
       mixins: "builderless"
     }
-    builders { mixins: "linux-optional-gpu-try" name: "linux_optional_gpu_tests_rel" }
+    builders { mixins: "linux-optional-gpu-try-trusty" name: "linux_optional_gpu_tests_rel" }
     builders {
       mixins: "linux-try"
       mixins: "upload_clang"