Add an API to fetch the WebView thread.

Some WebView APIs contain checks ensuring apps only use WebView APIs
from one single thread. The WebView Support Library should perform
similar thread checks. To do so we publish an API to get the WebView
thread, so that the support library knows which thread to check against.

Test: Ran the corresponding CTS tests.
Bug: 38220806
Change-Id: Ibab878e211d9594df6d39f99ce130e28fac7f019
diff --git a/api/current.txt b/api/current.txt
index 2dae343..b4d047d 100644
--- a/api/current.txt
+++ b/api/current.txt
@@ -50280,6 +50280,7 @@
     method public android.graphics.Bitmap getFavicon();
     method public android.webkit.WebView.HitTestResult getHitTestResult();
     method public deprecated java.lang.String[] getHttpAuthUsernamePassword(java.lang.String, java.lang.String);
+    method public android.os.Looper getLooper();
     method public java.lang.String getOriginalUrl();
     method public int getProgress();
     method public boolean getRendererPriorityWaivedWhenNotVisible();
diff --git a/core/java/android/webkit/WebView.java b/core/java/android/webkit/WebView.java
index 00860a4..2c51ee9 100644
--- a/core/java/android/webkit/WebView.java
+++ b/core/java/android/webkit/WebView.java
@@ -663,6 +663,10 @@
         if (context == null) {
             throw new IllegalArgumentException("Invalid context argument");
         }
+        if (mWebViewThread == null) {
+            throw new RuntimeException(
+                "WebView cannot be initialized on a thread that has no Looper.");
+        }
         sEnforceThreadChecking = context.getApplicationInfo().targetSdkVersion >=
                 Build.VERSION_CODES.JELLY_BEAN_MR2;
         checkThread();
@@ -2422,6 +2426,14 @@
         return getFactory().getWebViewClassLoader();
     }
 
+    /**
+     * Returns the {@link Looper} corresponding to the thread on which WebView calls must be made.
+     */
+    @NonNull
+    public Looper getLooper() {
+        return mWebViewThread;
+    }
+
     //-------------------------------------------------------------------------
     // Interface for WebView providers
     //-------------------------------------------------------------------------