Add Google Chrome variants

Casks/google-chrome-beta.rb

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+class GoogleChromeBeta < Cask
+  url 'http://dl.google.com/chrome/mac/beta/googlechrome.dmg'
+  homepage 'https://www.google.com/intl/en/chrome/browser/beta.html'
+  version 'beta-channel'
+  no_checksum
+  link 'Google Chrome.app'
+end

Casks/google-chrome-canary.rb

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+class GoogleChromeCanary < Cask
+  url 'https://storage.googleapis.com/chrome-canary/GoogleChromeCanary.dmg'
+  homepage 'https://tools.google.com/dlpage/chromesxs'
+  version 'latest'
+  no_checksum
+  link 'Google Chrome Canary.app'
+end

Casks/google-chrome-dev.rb

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+class GoogleChromeDev < Cask
+  url 'http://dl.google.com/chrome/mac/dev/googlechrome.dmg'
+  homepage 'https://www.google.com/chrome/'
+  version 'dev-channel'
+  no_checksum
+  link 'Google Chrome.app'
+end