[CrOS PhoneHub] Do not access SerializedNavigationEntry vector if empty.

Accessing an empty sessions::SessionTab's |navigations| vector is
resulting in an out of bounds access, causing
BrowserTabsMetadataFetcherImpl to crash.

Fixed: 1142562
Bug: 1106937
Change-Id: I0ec890135f6acafd61a39a7d06b52ef1956bed96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522497
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824826}

chrome/browser/chromeos/phonehub/browser_tabs_metadata_fetcher_impl.cc

@@ -24,6 +24,12 @@ GetSortedMetadataWithoutFavicons(const sync_sessions::SyncedSession* session) {
     const sessions::SessionWindow& window = window_pair.second->wrapped_window;
     for (const std::unique_ptr<sessions::SessionTab>& tab : window.tabs) {
       int selected_index = tab->normalized_navigation_index();
+
+      if (selected_index + 1 > tab->navigations.size() ||
+          tab->navigations.empty()) {
+        continue;
+      }
+
       const sessions::SerializedNavigationEntry& current_navigation =
           tab->navigations.at(selected_index);
 

chrome/browser/chromeos/phonehub/browser_tabs_metadata_fetcher_impl_unittest.cc

@@ -227,6 +227,17 @@ TEST_F(BrowserTabsMetadataFetcherImplTest, NoTabsOpen) {
 
   AttemptFetch();
   CheckIsExpectedMetadata({});
+
+  auto synced_session_window_two =
+      std::make_unique<sync_sessions::SyncedSessionWindow>();
+
+  // Add a tab without navigation(s), i.e no available metadata.
+  auto tab = std::make_unique<sessions::SessionTab>();
+  synced_session_window_two->wrapped_window.tabs.push_back(std::move(tab));
+  AddWindow(std::move(synced_session_window_two));
+
+  AttemptFetch();
+  CheckIsExpectedMetadata({});
 }
 
 TEST_F(BrowserTabsMetadataFetcherImplTest, BelowMaximumNumberOfTabs) {