Splitting the CL into parts with suggested approach

[ltilve]

From https://codereview.chromium.org/1152613003/#msg15

I think we should split it up into (at least) 3. The first one should include
no "real" UI code - that is, it shouldn't have anything touched in ui/views or
ui/cocoa, and you can probably leave out a little of the stuff in, e.g.,
ui/browser. The second should add the views implementation, and the third
should add the cocoa implementation.

[ltilve]

We have been splitting into a smaller set of buildable parts to be grouped into a reasonable patchgroup here https://github.com/ltilve/chromium/commits/igalia-sidebar-CL-splitted

[ltilve]

The initial issue https://codereview.chromium.org/1152613003 has been split for review as suggested into:

• https://codereview.chromium.org/1168383002/
• https://codereview.chromium.org/1169823005/
• https://codereview.chromium.org/1171873003/

[ltilve]

With all the additional modifications, the code is properly fragmented according to Devlin criteria. The pending modifications related with the tests to be tracked at #24