Modify iOSShell-gyp to generate unique bundle identifier

BUG=skia:

Review URL: https://codereview.chromium.org/1483143002
diff --git a/experimental/iOSShell/iOSShell-Info.plist b/experimental/iOSShell/iOSShell-Info.plist
index 69b62ef..7f1a827 100644
--- a/experimental/iOSShell/iOSShell-Info.plist
+++ b/experimental/iOSShell/iOSShell-Info.plist
@@ -11,7 +11,7 @@
 	<key>CFBundleIconFile</key>
 	<string></string>
 	<key>CFBundleIdentifier</key>
-	<string>com.google.iOSShell</string>
+	<string>$(PRODUCT_BUNDLE_IDENTIFIER)</string>
 	<key>CFBundleInfoDictionaryVersion</key>
 	<string>6.0</string>
 	<key>CFBundleName</key>