Use the return value to signal success for VTTParser helpers

Source/core/html/track/vtt/VTTCue.cpp

Index: Source/core/html/track/vtt/VTTCue.cpp
diff --git a/Source/core/html/track/vtt/VTTCue.cpp b/Source/core/html/track/vtt/VTTCue.cpp
index 7006b00981ad87ecf720b28d0141c468e9f2bb13..a0c47155775809e6b61cb9492185f7feca19b96f 100644
--- a/Source/core/html/track/vtt/VTTCue.cpp
+++ b/Source/core/html/track/vtt/VTTCue.cpp
@@ -675,8 +675,9 @@ void VTTCue::markFutureAndPastNodes(ContainerNode* root, double previousTimestam
         if (child->nodeName() == timestampTag) {
             unsigned position = 0;
             String timestamp = child->nodeValue();
-            double currentTimestamp = VTTParser::collectTimeStamp(timestamp, &position);
-            ASSERT(currentTimestamp != -1);
+            double currentTimestamp;
+            bool check = VTTParser::collectTimeStamp(timestamp, &position, currentTimestamp);
+            ASSERT_UNUSED(check, check);
 
             if (currentTimestamp > movieTime)
                 isPastNode = false;