[test] Prepare removing gpu defaults for tester bots

We want to remove the gpu:none default as we want to switch to
Mac Minis in the Mac pool that have gpus.

This starts a 3-way change:
1. This CL: Add the gpu dimension for Mac source side.
2. Remove setting it as default for Mac in infra.
3. Flip the value for gpu source side.

This requires merging to beta/stable.

No-Try: true
Bug: chromium:1174040
Change-Id: I81f2f5863593aa93fa668b4534d1116a11768f31
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2673402
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72532}

infra/testing/PRESUBMIT.py

@@ -29,6 +29,7 @@ SUPPORTED_SWARMING_DIMENSIONS = [
   'cpu',
   'device_os',
   'device_type',
+  'gpu',
   'os',
   'pool',
 ]

infra/testing/builders.pyl

@@ -647,6 +647,7 @@
     'swarming_dimensions' : {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'tests': [
       {'name': 'v8testing', 'shards': 4},
@@ -656,6 +657,7 @@
     'swarming_dimensions' : {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'tests': [
       {'name': 'mozilla'},
@@ -668,6 +670,7 @@
     'swarming_dimensions' : {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'tests': [
       {'name': 'd8testing', 'test_args': ['--gc-stress'], 'shards': 4},
@@ -677,6 +680,7 @@
     'swarming_dimensions' : {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'tests': [
       {'name': 'mozilla'},
@@ -719,6 +723,7 @@
     'swarming_dimensions' : {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'tests': [
       {'name': 'v8testing', 'shards': 8},
@@ -728,6 +733,7 @@
     'swarming_dimensions' : {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'tests': [
       {'name': 'v8testing', 'shards': 8},
@@ -737,6 +743,7 @@
     'swarming_dimensions' : {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'tests': [
       {'name': 'v8testing', 'shards': 8},
@@ -1300,6 +1307,7 @@
     'swarming_dimensions': {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'tests': [
       {'name': 'mozilla'},
@@ -1312,6 +1320,7 @@
     'swarming_dimensions': {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'tests': [
       {'name': 'mozilla'},
@@ -1324,6 +1333,7 @@
     'swarming_dimensions': {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'tests': [
       {'name': 'v8testing', 'shards': 5},
@@ -1333,6 +1343,7 @@
     'swarming_dimensions': {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'tests': [
       {'name': 'd8testing', 'test_args': ['--gc-stress'], 'shards': 4},
@@ -1358,6 +1369,7 @@
     'swarming_dimensions' : {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'swarming_task_attrs': {
       'expiration': 14400,
@@ -1372,6 +1384,7 @@
     'swarming_dimensions' : {
       'cpu': 'x86-64',
       'os': 'Mac-10.15',
+      'gpu': 'none',
     },
     'swarming_task_attrs': {
       'expiration': 14400,