Initial implementation of FeatureList in base/.

This first CL adds the initial version of FeatureList singleton class,
providing support for the following:
  - Initial class structure and singleton registration
  - Support for --enable-features= and --disable-features= flags
  - API to test if a feature is enabled or not
  - Its registration in chrome_browser_main.cc
  - Debug checks to ensure each Feature struct is defined only once
  - Basic unit tests

Parts that will be implemented in follow-up CLs:
  - Integration with FieldTrials (split out into a follow-up CL)
  - VariationsService integration and kill switch support
  - Various bits above base/ (e.g. about flags, sub-process stuff ,..)

BUG=526169

Review URL: https://codereview.chromium.org/1278403003

Cr-Commit-Position: refs/heads/master@{#347438}

• base/BUILD.gn[diff]
• base/OWNERS[diff]
• base/base.gyp[diff]
• base/base.gypi[diff]
• base/feature_list.cc[Added - diff]
• base/feature_list.h[Added - diff]
• base/feature_list_unittest.cc[Added - diff]
• chrome/browser/chrome_browser_main.cc[diff]
• chrome/common/chrome_switches.cc[diff]
• chrome/common/chrome_switches.h[diff]