Fix assert in grit script by always resolving template vars.

OutputNode.GetFilename() does not resolve environment vars, meaning that
when vars are used, the filename used in python does not match the files
created by the script. Use GetOutputFilename() everywhere, which properly
resolves variables in file names.

BUG=
Test: Build grit target with template vars in the output file name.

Review-Url: https://codereview.chromium.org/2802853003
Cr-Original-Commit-Position: refs/heads/master@{#464616}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6fcd1789ae575456137efd8e707003492d443751
3 files changed