Split OmniboxEditController into //components- and //chrome-level classes

Split OmniboxEditController into //components- and //chrome-level classes

OmniboxEditController is the base class via which the rest of the omnibox
cross-platform code interacts with platform-specific implementations. As such,
it is due to be componentized and shared with iOS.

However, it also has two aspects that do not make sense for sharing with iOS:
- A GetWebContents() virtual method [//content specific]
- Knowledge of CommandUpdater [//chrome specific]

This CL splits OmniboxEditController into two:
- A //components-level class, which has the base interface minus the above-
  mentioned aspects.
- A //chrome-level class, which adds the above aspects to the base interface.

The platform-specific implementations in //chrome are changed to inherit from
ChromeOmniboxEditController.

BUG=511944

Review URL: https://codereview.chromium.org/1254173005

Cr-Commit-Position: refs/heads/master@{#341571}