Fix 64 bit truncation on display_item.h

Assign the derived size after the SECURITY_DCHECKS to make it clear
we never expect this to exceed the size reserved.

BUG=879657

Change-Id: I1bb9f8724b9605499286935002a45508e58f2b04
Reviewed-on: https://chromium-review.googlesource.com/c/1347577Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611381}

third_party/blink/renderer/platform/graphics/paint/display_item.h

@@ -152,7 +152,6 @@ class PLATFORM_EXPORT DisplayItem {
         visual_rect_(client.VisualRect()),
         outset_for_raster_effects_(client.VisualRectOutsetForRasterEffects()),
         type_(type),
-        derived_size_(derived_size),
         fragment_(0),
         is_cacheable_(client.IsCacheable()),
         is_tombstone_(false) {
@@ -160,6 +159,7 @@ class PLATFORM_EXPORT DisplayItem {
     // If it doesn't, enlarge |derived_size_| and fix this assert.
     SECURITY_DCHECK(derived_size < (1 << 8));
     SECURITY_DCHECK(derived_size >= sizeof(*this));
+    derived_size_ = static_cast<unsigned>(derived_size);
   }
 
   virtual ~DisplayItem() = default;