PSA: changes to how branded strings and built and used

[Henrique Ferreiro]

As part of https://crbug.com/1470725, the targets for branded strings will be merged so that only the ones named `*chromium*` remain, and only the corresponding set of header files is generated. As an example, `//components/strings:google_chrome_strings` will be removed in favor of `//components/strings:components_chromium_strings`. But the latter will use either `components/components_google_chrome_strings.grd` or `components/components_chromium_strings.grd`, depending on the type of build by using the `branding_path_product` gn variable.

A follow-up CL will change the naming to `branded_strings`.

This work should simplify the changes required by Chromium forks, where only changing `branding_path_product` should be enough if the appropriate *.grd file is provided.

[Henrique Ferreiro]

Following the work at https://crbug.com/1470725, https://crrev.com/c/6178281 will rename pak files from *_${branding_path_product}_strings_$locale.pak to *branded_strings_$locale.pak. See an example at https://crrev.com/c/chromium/src/+/6178281/1/chrome/app/chromium_strings.grd.