Zero-check the AppBannerInfoBarDelegateAndroid native pointer.

There are a number of crashes which seem to come from the Java-side
AppBannerInfoBarDelegateAndroid#OnInstall method calling the native-side
method after its AppBannerUiDelegate object has been cleared. The crash
manifests as a segmentation fault when accessing a std::unique_ptr
member.

This CL zero-checks the native pointer in the Java-side
AppBannerInfoBarDelegateAndroid prior to calling to native.
https://crrev.com/c/959409 attempted to address the crashes by reordering
the destruction procedure of AppBannerInfoBarDelegateAndroid, but this
did not catch all of the crashes.

BUG=819434

Change-Id: Ibb5dbe322ab705173bba3835702fdd27b0e2adcc
Reviewed-on: https://chromium-review.googlesource.com/991312Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547876}

chrome/android/java/src/org/chromium/chrome/browser/infobar/AppBannerInfoBarDelegateAndroid.java

@@ -23,17 +23,17 @@ public class AppBannerInfoBarDelegateAndroid implements InstallerDelegate.Observ
 
     @Override
     public void onInstallIntentCompleted(InstallerDelegate delegate, boolean isInstalling) {
-        nativeOnInstallIntentReturned(mNativePointer, isInstalling);
+        if (mNativePointer != 0) nativeOnInstallIntentReturned(mNativePointer, isInstalling);
     }
 
     @Override
     public void onInstallFinished(InstallerDelegate delegate, boolean success) {
-        nativeOnInstallFinished(mNativePointer, success);
+        if (mNativePointer != 0) nativeOnInstallFinished(mNativePointer, success);
     }
 
     @Override
     public void onApplicationStateChanged(InstallerDelegate delegate, int newState) {
-        nativeUpdateInstallState(mNativePointer);
+        if (mNativePointer != 0) nativeUpdateInstallState(mNativePointer);
     }
 
     @CalledByNative