DCHECK that jstring to C++ string conversions don't pass in null.

android_webview/java/src/org/chromium/android_webview/AwContents.java

Index: android_webview/java/src/org/chromium/android_webview/AwContents.java
diff --git a/android_webview/java/src/org/chromium/android_webview/AwContents.java b/android_webview/java/src/org/chromium/android_webview/AwContents.java
index 046ccfccecb9e2b2281b33dc4b7e558345fc8cc6..4ddd359936de48507a4df36bdc2e179d2fc8d115 100644
--- a/android_webview/java/src/org/chromium/android_webview/AwContents.java
+++ b/android_webview/java/src/org/chromium/android_webview/AwContents.java
@@ -1347,6 +1347,14 @@ public class AwContents implements SmartClipProvider,
         ValueCallback<String[]> callback = new ValueCallback<String[]>() {
             @Override
             public void onReceiveValue(final String[] value) {
+                if (value != null) {
+                    // Replace null values with empty strings, because they can't be represented as
+                    // native strings.
+                    for (int i = 0; i < value.length; i++) {
+                        if (value[i] == null) value[i] = "";
+                    }
+                }
+
                 ThreadUtils.runOnUiThread(new Runnable() {
                     @Override
                     public void run() {