Switch remaining chromium.gpu.fyi Mac testers to Linux VMs.

Set up buildbucket for dawn-mac-x64-deps-rel trybot.

Bug: 950021
Bug: 948902
Change-Id: I0e64633e5f929821676e97039a8afb27644c858d
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1562617Reviewed-by: smut <smut@google.com>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649674}

infra/config/cr-buildbucket.cfg

@@ -1510,62 +1510,64 @@ buckets {
       name: "GPU FYI Mac dEQP Builder"
       mixins: "mac-gpu-fyi-ci"
     }
+    # Note that the Mac testers are all thin Linux VMs, triggering jobs on the
+    # physical Mac hardware in the Swarming pool, and therefore use the
+    # linux-dawn-ci mixin.
     builders {
       name: "Mac FYI 10.14 Release (AMD)"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
       mixins: "gpu-slow-bot"
     }
     builders {
       name: "Mac FYI 10.14 Release (Intel)"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
       mixins: "gpu-slow-bot"
     }
     builders {
       name: "Mac FYI 10.14 Release (NVIDIA)"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
       mixins: "gpu-slow-bot"
     }
     builders {
       name: "Mac FYI Release (Intel)"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
     }
     builders {
       name: "Mac FYI Debug (Intel)"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
     }
     builders {
       name: "Mac Pro FYI Release (AMD)"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
       mixins: "gpu-slow-bot"
     }
     builders {
       name: "Mac FYI Retina Release (NVIDIA)"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
     }
     builders {
       name: "Mac FYI Retina Debug (NVIDIA)"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
     }
     builders {
-      # TODO(kbr): testing running this on a thin Linux VM.
       name: "Mac FYI Retina Release (AMD)"
       mixins: "linux-gpu-fyi-ci"
     }
     builders {
       name: "Mac FYI Retina Debug (AMD)"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
     }
     builders {
       name: "Mac FYI Experimental Release (Intel)"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
     }
     builders {
       name: "Mac FYI Experimental Retina Release (AMD)"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
     }
     builders {
       name: "Mac FYI Experimental Retina Release (NVIDIA)"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
       # This bot has one machine backing its tests at the moment.
       # If it gets more, this should be switched back to gpu-slow-bot.
       # See crbug.com/853307 for more context.
@@ -1573,16 +1575,16 @@ buckets {
     }
     builders {
       name: "Mac FYI GPU ASAN Release"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
       execution_timeout_secs: 14400  # 4h
     }
     builders {
       name: "Mac FYI dEQP Release AMD"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
     }
     builders {
       name: "Mac FYI dEQP Release Intel"
-      mixins: "mac-gpu-fyi-ci"
+      mixins: "linux-gpu-fyi-ci"
     }
 
     # iOS bots.
@@ -3660,6 +3662,7 @@ buckets {
     builders { mixins: "ios-try" name: "ios-simulator-eg" }
     builders { mixins: "ios-try" name: "ios-simulator-xcode-clang" }
     builders { mixins: "ios-try" name: "ios-slimnav" }
+    builders { mixins: "mac-dawn-try" name: "dawn-mac-x64-deps-rel" }
     builders { mixins: "mac-angle-try" name: "mac-angle-rel" }
     builders { mixins: "mac-angle-try" name: "mac_angle_compile_dbg_ng" }
     builders { mixins: "mac-angle-try" name: "mac_angle_dbg_ng" }