Keep the reference to local private variable.

android_webview/apk/java/proguard.flags

 # Don't rename anything, it makes stack traces unintelligible. We only allow the
 # obfuscation pass to run so that we can discard attributes like local variable
 # tables. However, we don't want to lose out on optimizations, so although they
 # can occasionally make the stack trace a bit stranger with inlining, it won't
 # make a big difference for the users seeing the stacktraces. We can always use
 # 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 {
     public *;
 }
 
 -keep class com.android.webview.chromium.WebViewChromiumFactoryProviderFor* {
     public *;
 }
 
-
 -keep class com.android.webview.chromium.WebViewDatabaseAdapter {
   public *;
 }
 
 # This is the main entry point for APIs. It is kept to make developing with
 # unreleased Android easier.
 -keep class com.android.webview.chromium.WebViewChromium {
   public *;
 }
 
@@ skipping 38 matching lines @@
 -dontnote com.android.webview.chromium.AwSafeBrowsingApiHandler
 
 #TODO(hush): remove after N release. crbug.com/546762
 -keep class com.android.webview.chromium.ContentSettingsAdapter {
   public void setDisabledActionModeMenuItems(int);
   public int getDisabledActionModeMenuItems();
 }
 
 # We strip some unused resources when preprocessing the GMS client libs.
 -dontwarn com.google.android.gms.R**