Update fontconfig cache key handling

Since If6853404f1e87de4f95a24f6231282543814bd63, the "uuid" variable has
been referring to the MD5 of a fixed string. Update the comment as such,
and remove the now-unneeded .uuid file writing.

Also update test_fonts/README.chromium as the manual UUID change is no
longer necessary or effective.

Bug: 767754
Change-Id: I07a9b10a631e755877cbcb7df40a53359329f8a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209397
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Timothy Gu <timothygu@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770287}

base/test/generate_fontconfig_caches.cc

@@ -25,16 +25,14 @@
 // determinism. We have no way of guaranteeing that this produces correct
 // results, or even has the intended effect.
 int main() {
-  // fontconfig generates a random uuid and uses it to match font folders with
-  // the font cache. Rather than letting fontconfig generate a random uuid,
-  // which introduces build non-determinism, we place a fixed uuid in the font
-  // folder, which fontconfig will use to generate the cache.
   base::FilePath dir_module;
   base::PathService::Get(base::DIR_MODULE, &dir_module);
-  base::FilePath uuid_file_path =
-      dir_module.Append("test_fonts").Append(".uuid");
-  const char uuid[] = "fb5c91b2895aa445d23aebf7f9e2189c";
-  WriteFile(uuid_file_path, uuid);
+
+  // This is the MD5 hash of "/test_fonts", which is used as the key of the
+  // fontconfig cache.
+  //     $ echo -n /test_fonts | md5sum
+  //     fb5c91b2895aa445d23aebf7f9e2189c  -
+  const char cache_key[] = "fb5c91b2895aa445d23aebf7f9e2189c";
 
   // fontconfig writes the mtime of the test_fonts directory into the cache. It
   // presumably checks this later to ensure that the cache is still up to date.
@@ -61,6 +59,6 @@ int main() {
 
   // Check existence of intended fontconfig cache file.
   CHECK(base::PathExists(
-      fontconfig_caches.Append(base::StrCat({uuid, "-le64.cache-7"}))));
+      fontconfig_caches.Append(base::StrCat({cache_key, "-le64.cache-7"}))));
   return 0;
 }

third_party/test_fonts/README.chromium

@@ -27,9 +27,7 @@ How to update or add fonts:
 6.  Add all the font_bundle.tar.gz.sha1 file to the chromium src repository, by
     executing the following command:
 	$ git add ./third_party/test_fonts/test_fonts.tar.gz.sha1
-7.  Update filename constant for fontconfig cache in //base/test/BUILD.gn and
-    //base/test/generate_fontconfig_caches.cc when test fonts change.
-8.  (optional) Update FONT_FILES in
+7.  (optional) Update FONT_FILES in
     third_party/blink/tools/blinkpy/web_tests/port/base.py .
 
 If you need access to the chromium-fonts bucket, contact Chrome infra.% For