views: Fix linux omnibox colors by refactoring omnibox_result_view.cc.

Previously, colors in the omnibox were handled by checking if the result
of GetNativeTheme() was NativeThemeAura::instance(). However, this meant
in practice that there was an assumption that the native theme could
only be NativeThemeWin or NativeThemeAura. This assumption meant that we
did different things regarding omnibox colors on Win, Aura and Linux
Aura/GTK.

This patch adds color constants for the various colors used in the
omnibox. It then separates the existing initialization code into
NativeThemeWin and NativeThemeAura, as appropriate. Finally, it adds a
NativeThemeGtk2, which matches the current GTK port.

BUG=318484

Review URL: https://codereview.chromium.org/222613005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@261875 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed