Allow Offline MHTML save operations to succeed even with missing frames.

content/public/common/mhtml_generation_params.h

Index: content/public/common/mhtml_generation_params.h
diff --git a/content/public/common/mhtml_generation_params.h b/content/public/common/mhtml_generation_params.h
index 32825e49ddc888b12bf8c2f7ae63550eb2e7fe0f..0d7e6c2260b4564116cdfdff3f4961544cb99532 100644
--- a/content/public/common/mhtml_generation_params.h
+++ b/content/public/common/mhtml_generation_params.h
@@ -28,6 +28,11 @@ struct CONTENT_EXPORT MHTMLGenerationParams {
   // Cache-Control header is used.
   blink::WebFrameSerializerCacheControlPolicy cache_control_policy =
       blink::WebFrameSerializerCacheControlPolicy::None;
+
+  // This setting defaults to |false|. If set to |true| it allows the saving
+  // operation to still succeed even when frames go missing during the saving
+  // process (with the obvious exception of the main frame).
+  bool ignore_missing_frames = false;
 };
 
 }  // namespace content