Auto merge of #13432 - shinglyu:layout_viewer_clean, r=mbrubeck

Restore the layout trace viewer

<!-- Please describe your changes on the following line: -->
This patch restores the layout trace viewer removed in e7510ab90c , with some changes that makes it work with https://github.com/servo/servo/pull/13214

Sorry for the minified 3rd party CSS frameworks, it was from the original version and I haven't got time to replace them with hand-crafted CSS.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because is a stand-alone debugging tool

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python/servo/devenv_commands.py

@@ -154,7 +154,7 @@ class MachCommands(CommandBase):
         # Absolute paths for all directories to be considered
         grep_paths = root_dirs_abs + tests_dirs_abs
         return call(
-            ["git"] + ["grep"] + params + ['--'] + grep_paths + [':(exclude)*.min.js'],
+            ["git"] + ["grep"] + params + ['--'] + grep_paths + [':(exclude)*.min.js', ':(exclude)*.min.css'],
             env=self.build_env())
 
     @Command('fetch',