Update the Chromium Codelab

- New top-level //codelabs directory
- Update exercises 0-4 to compile
- Add mojo exercise

Previous codelab is here:
https://www.chromium.org/developers/cpp-in-chromium-101-codelab

Bug: N/A
Change-Id: I2218160fa9efc8017dce809db8546604ade7e5d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414488
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Austin Sullivan <asully@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814678}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 69af581..2e6e7ca 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -68,6 +68,8 @@
     r'ios[\\/].*_app_interface\.mm$',
     # Views Examples code
     r'ui[\\/]views[\\/]examples[\\/].*',
+    # Chromium Codelab
+    r'codelabs[\\/]*'
 )
 
 _THIRD_PARTY_EXCEPT_BLINK = 'third_party/(?!blink/)'