Add a script to GRIT to generate predetermined resource ids file.

tools/gritsettings/README.md

Index: tools/gritsettings/README.md
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+### tools/gritsettings README
+
+This directory contains several files that apply global to the Chrome resource
+generation system (which uses GRIT - see tools/grit).
+
+**resource_ids**: This file is used to assign starting resource ids for
+resources and strings used by Chromium. This is done to ensure that resource ids
+are unique across all the grd files. If you are adding a new grd file, please
+add a new entry to this file.
+
+**translation_expectations.pyl**: Specifies which grd files should be translated
+and into which languages they should be translated. Used by the internal
+translation process.
+
+**startup_resources_[platform].txt**: These files provide a pre-determined
+resource id ordering that will be used by GRIT when assigning resources ids. The
+goal is to have the resource loaded during Chrome startup be ordered first in
+the .pak files, so that fewer page faults are suffered during Chrome start up.
+To update or generate one of these files, follow these instructions:
+
+  1. Build a Chrome official release build and launch it with command line:
+     `--print-resource-ids` and save the output to a file (e.g. res.txt).
+
+  2. Generate the startup_resources_[platform].txt via the following command
+     (you can redirect its output to the new file location):
+
+     `
+     tools/grit/grit/format/gen_predetermined_ids.py res_ids.txt out/gn
+     `
+
+     In the above command, res_ids.txt is the file produced in step 1 and out/gn
+     is you Chrome build directory where you compiled Chrome. The output of the
+     command can be added as a new startup_resource_[platform]
+
+  3. If this is a new file, modify `tools/grit/grit_rule.gni` to set its path
+     via `grit_predetermined_resource_ids_file` for the given platform.