Add a TimeUntilNextTry to reading list

ios/chrome/browser/reading_list/reading_list_entry_unittest.cc

Index: ios/chrome/browser/reading_list/reading_list_entry_unittest.cc
diff --git a/ios/chrome/browser/reading_list/reading_list_entry_unittest.cc b/ios/chrome/browser/reading_list/reading_list_entry_unittest.cc
index 215c3fdd32df3c9a0d0ff76facae81c995ee8fd4..4deef7786ecea0d5d624fee58f881727ed781bf6 100644
--- a/ios/chrome/browser/reading_list/reading_list_entry_unittest.cc
+++ b/ios/chrome/browser/reading_list/reading_list_entry_unittest.cc
@@ -50,3 +50,23 @@ TEST(ReadingListEntry, DistilledState) {
   e.SetDistilledURL(distilled_url);
   EXPECT_EQ(ReadingListEntry::PROCESSED, e.DistilledState());
 }
+
+// Tests the auto incrementation of the failed distillation counter.
+TEST(ReadingListEntry, FailedDistillationCounter) {
+  ReadingListEntry e(GURL("http://example.com"), "bar");
+
+  ASSERT_EQ(0, e.FailedDistillationCounter());
+
+  e.SetDistilledState(ReadingListEntry::ERROR);
+  EXPECT_EQ(1, e.FailedDistillationCounter());
+
+  e.SetDistilledState(ReadingListEntry::WILL_RETRY);

[noyau (Ping after 24h), 2016/09/16 14:25:13]

Is there any case where a transition from ERROR to WILL_RETRY is possible? Or
desirable? Should it be DCHECKed instead?

[gambard, 2016/09/19 09:08:09]

Potentially, if you launch two download before the first of the two has ended,
one finishing in ERROR and the other in WILL_RETRY. I don't know if it is
possible, but I don't want to prevent it.

+  EXPECT_EQ(1, e.FailedDistillationCounter());
+
+  e.SetDistilledState(ReadingListEntry::PROCESSING);
+  e.SetDistilledState(ReadingListEntry::WILL_RETRY);
+  EXPECT_EQ(2, e.FailedDistillationCounter());
+
+  e.SetDistilledState(ReadingListEntry::ERROR);
+  EXPECT_EQ(2, e.FailedDistillationCounter());
+}