Keep the reference to local private variable.

android_webview/apk/java/proguard.flags

Index: android_webview/apk/java/proguard.flags
diff --git a/android_webview/apk/java/proguard.flags b/android_webview/apk/java/proguard.flags
index dbbc1bb263e1023211bb722b8c666b43968ef1e9..495d48612b206412744f05995b6b73d480fe0c82 100644
--- a/android_webview/apk/java/proguard.flags
+++ b/android_webview/apk/java/proguard.flags
@@ -6,6 +6,10 @@
 # build/android/stacktrace/java_deobfuscate.py to fix the stacktrace up for us.
 -keepnames,allowoptimization class *** { *; }
 
+-keepclassmembers class org.chromium.android_webview.AwPdfExporter {
+    android.view.ViewGroup mContainerView;
+}
+
 # Keep the factory and its public members; it's the main entry point used by the
 # framework.
 -keep class com.android.webview.chromium.WebViewChromiumFactoryProvider {
@@ -16,7 +20,6 @@
     public *;
 }
 
-
 -keep class com.android.webview.chromium.WebViewDatabaseAdapter {
   public *;
 }