Switch to ValueOrDie

We know the values are always valid at this point, so use ValueOrDie instead
of ValueOrDefault.

BUG=chromium:637984

Review-Url: https://codereview.chromium.org/2319343003
diff --git a/core/fpdfapi/fpdf_render/fpdf_render_pattern.cpp b/core/fpdfapi/fpdf_render/fpdf_render_pattern.cpp
index afcaa8d..63c67cc 100644
--- a/core/fpdfapi/fpdf_render/fpdf_render_pattern.cpp
+++ b/core/fpdfapi/fpdf_render/fpdf_render_pattern.cpp
@@ -1149,8 +1149,8 @@
         if (!safeStartX.IsValid() || !safeStartY.IsValid())
           return;
 
-        start_x = safeStartX.ValueOrDefault(0);
-        start_y = safeStartY.ValueOrDefault(0);
+        start_x = safeStartX.ValueOrDie();
+        start_y = safeStartY.ValueOrDie();
       }
       if (width == 1 && height == 1) {
         if (start_x < 0 || start_x >= clip_box.Width() || start_y < 0 ||