Error while building 0.65.x

[bsclifton]

Description

Can be seen on Windows when building on 0.65.x

Error looks like this:

06:36:21  In file included from C:\jenkins\workspace\os_theme_with_brave_theme_0.65.x\src\brave\chromium_src\ui\base\webui\web_ui_util.cc:15:
06:36:21  ../../brave/chromium_src\chrome/grit/generated_resources.h(2,10):  fatal error: '../gen/chrome/grit/generated_resources.h' file not found
06:36:21  #include "../gen/chrome/grit/generated_resources.h"
06:36:21           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
06:36:21  1 error generated.

Steps to Reproduce

1. On Windows, clone brave-browser
2. Checkout 0.65.x branch
3. Update brave-core branch to be pr2219_align_windows_os_theme_with_brave_theme_0.65.x
4. Run npm run init and npm run build -- Release

Actual result:

Error occurs during compilation. For more info, see https://staging.ci.brave.com/job/brave-browser-build-pr/job/pr2219_align_windows_os_theme_with_brave_theme_0.65.x/1/execution/node/222/log/

Expected result:

No error

Reproduces how often:

Seems to reproduce in 0.65.x on CI... but doesn't reproduce on master (even though it's an issue there too)

Brave version (brave://version info)