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Output is a JSON file containing information about targets. The generator
can also optionally invoke a python script on generated file.

Example:
gn gen --ide=json ./out-json --json-ide-script=//scripts/custom-ide-generator.py \
    --ison-ide-script-args="additional script arguments"

Also implements --format=json for gn desc as described in https://bugs.chromium.org/p/chromium/issues/detail?id=620132

BUG=

Review-Url: https://codereview.chromium.org/2064533002
Cr-Commit-Position: refs/heads/master@{#406064}