Bug 1309613 - enable automigration on beta 51 for a few cycles, disable eslint checks for browser/branding/ pref files, r=dolske,me,a=gchang

MozReview-Commit-ID: KArvfbKkRDd

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extra : rebase_source : 9faed28a1777e95724fcc521a8f7c95e27deb47c
extra : intermediate-source : a945f68d5a5a5e3999f250e6b15fd991aeb1d33e
extra : source : 246ef504499551de19911e55490f35b7ad15f32b