Add new class RLZTrackerDelegate that abstract access to embedder
specific singletons and informations and provide an implementation for
Chrome based on the previous implementation.

Split rlz_unittest.cc in tests of the RLZTracker and Chrome
implementation of the RLZTrackerDelegate interface and move the
RLZTracker tests into the component.

Add a new gyp/gn variable "enable_rlz_support". This variable is true on
the platforms that support RLZ (currently Windows, Mac, iOS and
ChromeOS). Use it to build library and unit tests even when the RLZ
support is not enabled in the Chrome binary (this is still controlled by
"enable_rlz" and depends on the branding).

Enable the tests on iOS and convert rlz_tracker_ios.mm to a C++ file.

TEST=Run unit_tests and components_unittests on a platform that supports
RLZ (Windows, Mac, iOS or ChromeOS) and check that they pass. Then build
Chrome with "enable_rlz" and manually checks that RLZ are sent with
searches as expected.

BUG=504841,508148

Review URL: https://codereview.chromium.org/1212163011

Cr-Commit-Position: refs/heads/master@{#339002}