Replace uses of "master" in DIR_METADATA links with "HEAD"

This will still work after git "master" branch is renamed to "main",
and makes inclusive language dashboards easier to interpret right now.

Bug: none
Change-Id: I628be29ce14257b82aa0f88448a9650b56671359
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493186Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Zork <zork@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820374}

extensions/browser/api/automation_internal/DIR_METADATA

 # Metadata information for this directory.
 #
 # For more information on DIR_METADATA files, see:
-#   https://source.chromium.org/chromium/infra/infra/+/master:go/src/infra/tools/dirmd/README.md
+#   https://source.chromium.org/chromium/infra/infra/+/HEAD:go/src/infra/tools/dirmd/README.md
 #
 # For the schema of this file, see Metadata message:
-#   https://source.chromium.org/chromium/infra/infra/+/master:go/src/infra/tools/dirmd/proto/dir_metadata.proto
+#   https://source.chromium.org/chromium/infra/infra/+/HEAD:go/src/infra/tools/dirmd/proto/dir_metadata.proto
 
 monorail {
   component: "UI>Accessibility"
-}
\ No newline at end of file
+}

extensions/renderer/api/automation/DIR_METADATA

 # Metadata information for this directory.
 #
 # For more information on DIR_METADATA files, see:
-#   https://source.chromium.org/chromium/infra/infra/+/master:go/src/infra/tools/dirmd/README.md
+#   https://source.chromium.org/chromium/infra/infra/+/HEAD:go/src/infra/tools/dirmd/README.md
 #
 # For the schema of this file, see Metadata message:
-#   https://source.chromium.org/chromium/infra/infra/+/master:go/src/infra/tools/dirmd/proto/dir_metadata.proto
+#   https://source.chromium.org/chromium/infra/infra/+/HEAD:go/src/infra/tools/dirmd/proto/dir_metadata.proto
 
 monorail {
   component: "UI>Accessibility"
-}
\ No newline at end of file
+}

extensions/renderer/resources/automation/DIR_METADATA

 # Metadata information for this directory.
 #
 # For more information on DIR_METADATA files, see:
-#   https://source.chromium.org/chromium/infra/infra/+/master:go/src/infra/tools/dirmd/README.md
+#   https://source.chromium.org/chromium/infra/infra/+/HEAD:go/src/infra/tools/dirmd/README.md
 #
 # For the schema of this file, see Metadata message:
-#   https://source.chromium.org/chromium/infra/infra/+/master:go/src/infra/tools/dirmd/proto/dir_metadata.proto
+#   https://source.chromium.org/chromium/infra/infra/+/HEAD:go/src/infra/tools/dirmd/proto/dir_metadata.proto
 
 monorail {
   component: "UI>Accessibility"
-}
\ No newline at end of file
+}