Reland "Add expectations.txt validation to presubmit"

This is a reland of commit cde5589953

Original change's description:
> Add expectations.txt validation to presubmit
>
> Change-Id: Ibabdfbdf44b027a6e6441e140f23b2c2488f32dc
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/112120
> Reviewed-by: Ben Clayton <bclayton@google.com>
> Commit-Queue: Austin Eng <enga@chromium.org>
> Kokoro: Kokoro <noreply+kokoro@google.com>

Change-Id: I013eeec03e27b3147023ea094f5bde011561edd1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/113540
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>
Kokoro: Austin Eng <enga@chromium.org>

go_presubmit_support.py

@@ -12,17 +12,24 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+
+def go_path(input_api):
+    go = input_api.os_path.join(input_api.change.RepositoryRoot(), "tools",
+                                "golang", "bin", "go")
+    if input_api.is_windows:
+        go += '.exe'
+
+    return go
+
+
 def RunGoTests(input_api, output_api):
     results = []
     try:
-        go = input_api.os_path.join(input_api.change.RepositoryRoot(), "tools",
-                                    "golang", "bin", "go")
-        if input_api.is_windows:
-            go += '.exe'
-        input_api.subprocess.check_call_out([go, "test", "./..."],
-                                            stdout=input_api.subprocess.PIPE,
-                                            stderr=input_api.subprocess.PIPE,
-                                            cwd=input_api.PresubmitLocalPath())
+        input_api.subprocess.check_call_out(
+            [go_path(input_api), "test", "./..."],
+            stdout=input_api.subprocess.PIPE,
+            stderr=input_api.subprocess.PIPE,
+            cwd=input_api.PresubmitLocalPath())
     except input_api.subprocess.CalledProcessError as e:
         results.append(output_api.PresubmitError('%s' % (e, )))
     return results