Steps to reproduce the problem:
Create BUILD.gn script for building an APK, android_apk("your_apk").

Use android_assets("some_assets_name") as one of the dependencies of the aforementioned APK. Specify one source; I used:
android_assets("PolqaCalculatorMobile_assets") {
  sources = [
    "app/assets/",
  ]
}

Inside of the directory, have another sub-directory, and a number of files in it. For example:
app/assets/sub_dir/file_1
app/assets/sub_dir/file_2
app/assets/sub_dir/file_3

After building the APK, peek inside it. The directory structure created would be:
assets/ub_dir/file_1
assets/ub_dir/file_2
assets/ub_dir/file_3
(Namely, the "s" from "sub_dir" will be missing.

What is the expected behavior?
Unmodified directory structure.

What went wrong?
One letter was truncated from the very beginning of the output path.

Did this work before? N/A 

Chrome version: 57.0.2987.133  Channel: stable
OS Version: 
Flash Version: 

I've found the solution and fixed; I'll associated the CL with this bug.