Improve extension icon prediction

BUG=394920

Review URL: https://codereview.chromium.org/415813003

Cr-Commit-Position: refs/heads/master@{#289062}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289062 0039d316-1c4b-4281-b951-d872f2087c98

• chrome/browser/extensions/active_script_controller.cc[diff]
• chrome/browser/extensions/extension_action_manager.cc[diff]
• chrome/browser/extensions/extension_action_manager.h[diff]
• chrome/browser/extensions/extension_action_manager_unittest.cc[Added - diff]
• chrome/chrome_tests_unit.gypi[diff]