Fix mirror mode restoring after resume

Background:
Upon suspend, DisplayManager::SetSoftwareMirroring(true) will be called to
clear current mirroring source and destination displays. But mirror mode
is not enabled because DisplayManager::OnNativeDisplaysChanged() will
receive empty display info and stop before reconfiguration. Later upon
resume, mirror mode will not be restored because the mirror info is cleared.

Changes:
Prevent DisplayManager::SetSoftwareMirroring(true) from clearing the
mirroring source and destination displays as they will be cleared later
in DisplayManager::OnNativeDisplaysChanged().

Bug: 797016
Change-Id: I3b987e5bc13a72313f5273262e96ed26b7ca347c
Reviewed-on: https://chromium-review.googlesource.com/857548
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532505}

ash/display/display_manager_unittest.cc

@@ -4405,4 +4405,32 @@ TEST_F(DisplayManagerTest, MixedMirrorModeRestore) {
   EXPECT_EQ(first_display_id, destination_ids[0]);
 }
 
+TEST_F(DisplayManagerTest, MirrorModeRestoreAfterResume) {
+  const int64_t internal_display_id =
+      display::test::DisplayManagerTestApi(display_manager())
+          .SetFirstDisplayAsInternalDisplay();
+  constexpr int64_t external_display_id = 210000001;
+  std::vector<display::ManagedDisplayInfo> display_info_list;
+  display_info_list.emplace_back(display::CreateDisplayInfo(
+      internal_display_id, gfx::Rect(0, 0, 100, 100)));
+  display_info_list.emplace_back(display::CreateDisplayInfo(
+      external_display_id, gfx::Rect(1, 1, 500, 500)));
+
+  // Turn on mirror mode.
+  display_manager()->OnNativeDisplaysChanged(display_info_list);
+  display_manager()->SetMirrorMode(display::MirrorMode::kNormal, base::nullopt);
+  EXPECT_TRUE(display_manager()->IsInMirrorMode());
+
+  // Suspend.
+  display_manager()->SetMultiDisplayMode(display::DisplayManager::MIRRORING);
+  display_manager()->OnNativeDisplaysChanged(
+      std::vector<display::ManagedDisplayInfo>());
+  EXPECT_TRUE(display_manager()->IsInMirrorMode());
+
+  // Resume.
+  display_manager()->SetMultiDisplayMode(display::DisplayManager::MIRRORING);
+  display_manager()->OnNativeDisplaysChanged(display_info_list);
+  EXPECT_TRUE(display_manager()->IsInMirrorMode());
+}
+
 }  // namespace ash

ui/display/manager/display_manager.cc

@@ -867,17 +867,17 @@ void DisplayManager::OnNativeDisplaysChanged(
         [](const ManagedDisplayInfo& display_info) {
           return display_info.id();
         });
-    multi_display_mode_ = (base::CommandLine::ForCurrentProcess()->HasSwitch(
-                               ::switches::kEnableSoftwareMirroring) ||
-                           ShouldSetMirrorModeOn(list))
-                              ? MIRRORING
-                              : EXTENDED;
+    bool should_enable_software_mirroring =
+        base::CommandLine::ForCurrentProcess()->HasSwitch(
+            ::switches::kEnableSoftwareMirroring) ||
+        ShouldSetMirrorModeOn(list);
+    SetSoftwareMirroring(should_enable_software_mirroring);
   }
 #endif
 
   // Do not clear current mirror state before calling ShouldSetMirrorModeOn()
   // as it depends on the state.
-  software_mirroring_display_list_.clear();
+  ClearMirroringSourceAndDestination();
   hardware_mirroring_display_id_list_ = hardware_mirroring_display_id_list;
   mirroring_source_id_ = mirroring_source_id;
   num_connected_displays_ = updated_displays.size();
@@ -1517,7 +1517,6 @@ void DisplayManager::SetDefaultMultiDisplayModeForCurrentDisplays(
 
 void DisplayManager::SetMultiDisplayMode(MultiDisplayMode mode) {
   multi_display_mode_ = mode;
-  ClearMirroringSourceAndDestination();
 }
 
 void DisplayManager::ReconfigureDisplays() {