[dirmd] Use metadata mixins

Use the new "mixins" feature in dirmd.
Derive mixins from file:// links in OWNERS files.

This CL is machine-generated, in two parts.

Part 1: use mixins.

1. Save current metadata to JSON files, in ORIGINAL and REDUCED forms.
   dirmd read -form original > ~/tmp/dirmd/original.json
   dirmd read -form reduced  > ~/tmp/dirmd/reduced.json
2. Run mixins.py.
   Source code: https://gist.github.com/nodirg/b59d3df338a72a8944f957c5b3a15e22

The script finds related directories, connected by file:// links
in OWNERS files. For those directories that have the same base
name, e.g.
  components/autofill
  chrome/android/java/src/org/chromium/chrome/browser/autofill
if importing the "parent" to the "child" reduces metadata, or adds
missing metadata, extract the common metadata to COMMON_METADATA
and import it in both directories.

Part2: deduplicate metadata.
Apply same instructions as in
https://chromium-review.googlesource.com/c/chromium/src/+/2795985/16

Bug: 1179786
Change-Id: I2e6378004ce433e02f1c9360b09eb72354083bf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2897943
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Fred Mello <fredmello@chromium.org>
Owners-Override: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Cr-Commit-Position: refs/heads/main@{#925648}

Author: nodirt

apps/COMMON_METADATA

@@ -0,0 +1,4 @@
+monorail {
+  component: "Platform>Apps"
+}
+team_email: "apps-dev@chromium.org"

apps/DIR_METADATA

@@ -6,7 +6,4 @@
 # For the schema of this file, see Metadata message:
 #   https://source.chromium.org/chromium/infra/infra/+/main:go/src/infra/tools/dirmd/proto/dir_metadata.proto
 
-monorail {
-  component: "Platform>Apps"
-}
-team_email: "apps-dev@chromium.org"
+mixins: "//apps/COMMON_METADATA"