Bug 929122 - Add b2g-debug.json, r=ahal

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rename : testing/mochitest/b2g.json => testing/mochitest/b2g-debug.json
Bug 928900 - Reduce some content/ #includes; r=jst

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extra : rebase_source : d2932ff888affe7e1e7a10233dfacac6fb9f07a7