chromeos: Cleanup comments and DEPS in //ash

Remove references to //ash/common, rewrite the top-level README.md, and
remove unnecessary DEPS.

BUG=none
TEST=checkdeps passes

Review-Url: https://codereview.chromium.org/2811643003
Cr-Commit-Position: refs/heads/master@{#463301}

ash/README.md

+Ash
+---
+Ash is the "Aura Shell", the window manager and system UI for Chrome OS.
+Ash uses the views UI toolkit (e.g. views::View, views::Widget, etc.) backed
+by the aura native widget and layer implementations.
+
+Ash sits below chrome in the dependency graph (i.e. it cannot depend on code
+in //chrome). It has a few dependencies on //content, but these are isolated
+in their own module in //ash/content. This allows targets like ash_unittests
+to build more quickly.
+
+Tests
+-----
+Most tests should be added to the ash_unittests target. Tests that rely on
+//content should be added to ash_content_unittests, but these should be rare.
+
+Tests can bring up most of the ash UI and simulate a login session by deriving
+from AshTestBase. This is often needed to test code that depends on ash::Shell
+and the controllers it owns.
+
 Mus+ash
 ----------
-Ash is transitioning from using aura to using mus. During the
-transition period ash has support for both aura and mus. In order to
-work with both toolkits ash has a porting layer. This layer exists in
-ash/common. As portions of ash are converted to the porting layer they
-should move to ash/common. DEPS files may also be used to ensure new
-dependencies do not get added.
+Ash is transitioning to use the mus window server and gpu process, found in
+//services/ui. Ash continues to use aura, but aura is backed by mus. Code to
+support mus is found in //ash/mus. There should be relatively few differences
+between the pure aura and the aura-mus versions of ash. Ash can by run in mus
+mode by passing the --mus command line flag.
+
+Ash is also transitioning to run as a mojo service in its own process. This
+means that code in chrome cannot call into ash directly, but must use the mojo
+interfaces in //ash/public/interfaces.
+
+Out-of-process Ash is referred to as "mash" (mojo ash). In-process ash is
+referred to as "classic ash". Ash can run in either mode depending on the
+--mash command line flag.
+
+Historical notes
+----------------
+Ash shipped on Windows for a couple years to support Windows 8 Metro mode.
+Windows support was removed in 2016.

ash/accelerators/accelerator_controller_delegate_aura.h

@@ -14,9 +14,10 @@ namespace ash {
 
 class ScreenshotDelegate;
 
-// Contains all the accelerators that have dependencies on portions of ash
-// not yet converted to ash/common. When adding a new accelerator that only
-// depends on ash/common code, add it to accelerator_controller.cc instead.
+// Support for accelerators that only work in classic ash and not in mash,
+// for example accelerators related to display management. These sorts of
+// accelerators should be rare. Most new accelerators should be added to
+// accelerator_controller.cc instead.
 class ASH_EXPORT AcceleratorControllerDelegateAura
     : public AcceleratorControllerDelegate {
  public:

ash/mus/DEPS

@@ -10,16 +10,8 @@ include_rules = [
   "+services/service_manager/public",
   "+services/tracing/public",
 ]
-# TODO(jamescook): Are these still needed now that //ash/common is gone?
-specific_include_rules = {
-  "root_window_controller.cc": [
-    "+ash/shelf/shelf_layout_manager.h",
-  ],
-  "shelf_delegate_mus.cc": [
-    "+ash/shelf/shelf.h",
-    "+ash/shelf/shelf_widget.h",
-  ],
 
+specific_include_rules = {
   "app_launch_unittest.cc": [
     "+mash/quick_launch/public",
   ],

ash/test/ash_test.h

@@ -55,7 +55,7 @@ class WindowOwner {
 
 // Base class for ash tests. This class calls through to AshTestImpl for the
 // real implementation. This class exists so that tests can be written to
-// ash/common and run in both mus and aura.
+// run in both mus and aura.
 //
 // TODO: this class is deprecated and should be removed.
 // http://crbug.com/707057.

ash/wm/workspace/workspace_layout_manager_keyboard_unittest.cc

@@ -46,8 +46,6 @@ WorkspaceLayoutManager* GetWorkspaceLayoutManager(WmWindow* container) {
 
 }  // namespace
 
-// TODO: this is a duplicate of that in ash/common, and should move once
-// keyboard is refactored to work in mash.
 class WorkspaceLayoutManagerKeyboardTest : public AshTest {
  public:
   WorkspaceLayoutManagerKeyboardTest() : layout_manager_(nullptr) {}