Fix intermittent SpeedReaderBrowserTest.ShowOriginalPage failure (#35718)

WaitDistilled() was called without checking its return value.
If distillation times out or fails, the state doesn't reach
Distilled, but the test proceeds to look for a DOM element
that only exists in distilled HTML, resulting in a misleading
timeout error.

All other calls to WaitDistilled() in this file use
ASSERT_TRUE(WaitDistilled()), which catches the failure early
with a clear error message.

Resolves brave/brave-browser#54582

browser/speedreader/speedreader_browsertest.cc

@@ -652,8 +652,7 @@ IN_PROC_BROWSER_TEST_F(SpeedReaderBrowserTest, ReloadContent) {
 IN_PROC_BROWSER_TEST_F(SpeedReaderBrowserTest, ShowOriginalPage) {
   EnableSpeedreaderAllowedForAllSites();
   NavigateToPageSynchronously(kTestPageReadable);
-  // Wait for distillation to complete before interacting with distilled content
-  WaitDistilled();
+  ASSERT_TRUE(WaitDistilled());
   auto* web_contents = ActiveWebContents();
 
   static constexpr char kCheckNoApiInMainWorld[] =