Preprocessing: Ensure all files are written under the preprocessing dir

This will prevent possible collisions, and ensures that files generated
by a target all live in that target's output.

Note that unpack_pak.py, previously used to create the folder of
preprocessed Web UI files, has an equivalent assert.

Bug: 1071641
Change-Id: Ic24e2a6c98cc35957c692bfa1a0d40fca1b7423e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424846
Auto-Submit: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809511}

tools/grit/preprocess_grit.py

@@ -72,6 +72,8 @@ def main(argv):
 
     out_path = os.path.join(out_folder, input_file)
     out_dir = os.path.dirname(out_path)
+    assert out_dir.startswith(out_folder), \
+           'Cannot preprocess files to locations not under %s.' % out_dir
     try:
       os.makedirs(out_dir)
     except OSError as e: