Remove gc root from PowerSaveBlocker

Views used by PowerSaveBlocker should be allowed to be garbage
collected. WebView relies on this in order to not leak.

Bug: 1004166
Change-Id: I95a996f4de21ab437650cb4ac039adb56e21815e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1816797
Commit-Queue: Bo <boliu@chromium.org>
Auto-Submit: Bo <boliu@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698497}

services/device/wake_lock/power_save_blocker/android/java/src/org/chromium/device/power_save_blocker/PowerSaveBlocker.java

@@ -10,13 +10,13 @@ import org.chromium.base.annotations.CalledByNative;
 import org.chromium.base.annotations.JNINamespace;
 
 import java.lang.ref.WeakReference;
-import java.util.HashMap;
+import java.util.WeakHashMap;
 
 @JNINamespace("device")
 class PowerSaveBlocker {
     // Counter associated to a view to know how many PowerSaveBlocker are
-    // currently registered.
-    private static HashMap<View, Integer> sBlockViewCounter = new HashMap<View, Integer>();
+    // currently registered. Using WeakHashMap to prevent leaks in Android WebView.
+    private static WeakHashMap<View, Integer> sBlockViewCounter = new WeakHashMap<View, Integer>();
 
     // WeakReference to prevent leaks in Android WebView.
     private WeakReference<View> mKeepScreenOnView;
@@ -54,6 +54,9 @@ class PowerSaveBlocker {
         View view = mKeepScreenOnView.get();
         mKeepScreenOnView = null;
 
+        // View has been garbage collected. No need to worry about clean up.
+        if (view == null) return;
+
         Integer prev_counter = sBlockViewCounter.get(view);
         assert prev_counter != null;
         assert prev_counter.intValue() > 0;