Chromium's BUILD files try to avoid uses of exec_script when possible
because they slow down every GN invocation. In preparation for building
Dawn inside Chromium, the calls to exec_script for the code generator
are removed.
In GN, the generator now outputs a "JSON tarball", a dictionnary mapping
filenames to content. This allows us to use the "depfile" feature of GN
to avoid the exec_script call to gather the script's inputs.
Outputs of the generator are now listed in the BUILD.gn files. To keep
it in sync with the generator, GN outputs a file containing "expected
outputs" that is checked by the code generator.
Finally the dawn_generator GN template doesn't create a target anymore,
but users are expected to gather outputs using get_target_outputs.
Corentin Wallez