Add diagnostic message for distiller crash

This should not increase the crash rate.

Bug: 718153
Change-Id: I86915c51945db21cd5e528d7270150e648fbfc1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1810035
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Auto-Submit: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697717}

components/dom_distiller/core/distiller.cc

@@ -167,7 +167,14 @@ void DistillerImpl::OnPageDistillationFinished(
   }
 
   DCHECK(distiller_result);
+  CHECK_LT(started_pages_index_[page_num], pages_.size())
+      << "started_pages_index_[" << page_num
+      << "] (=" << started_pages_index_[page_num] << ") is out of range.";
   DistilledPageData* page_data = GetPageAtIndex(started_pages_index_[page_num]);
+  CHECK(page_data) << "GetPageAtIndex(started_pages_index_[" << page_num
+                   << "] (=" << started_pages_index_[page_num]
+                   << ")) returns nullptr. pages_.size() = " << pages_.size()
+                   << ".";
   page_data->distilled_page_proto =
       new base::RefCountedData<DistilledPageProto>();
   page_data->page_num = page_num;