History is in the process of becoming a layered compoent
(https://sites.google.com/a/chromium.org/dev/developers/design-documents/layered-components-design)
to enable it to be shared cleanly on iOS.

When this process is complete, this component will have the following structure:

core/: shared code that does not depends on src/content/ or src/ios/
  browser/: browser process code
  common/: code shared by the browser and the renderer
content/: driver for the shared code based on the content layer
  browser/: browser process code
  common/: code shared by the browser and the renderer
ios/: driver for the shared code based on src/ios