[FlexNG] DCHECK that flex base border box includes border+padding

We can do this now that issue 1094311 is fixed.

The new DCHECK is triggered by issue 1102183 earlier than the one in the
report. The real fix for this issue will come later.

Bug: 1102183
Change-Id: I5235ece346e31724c103a3fb53a5bf920ec8a657
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2311040
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Auto-Submit: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791104}

third_party/blink/renderer/core/layout/ng/flex/ng_flex_layout_algorithm.cc

@@ -609,12 +609,9 @@ void NGFlexLayoutAlgorithm::ConstructAndAppendFlexItems() {
     // https://www.w3.org/TR/css-flexbox-1/#algo-main-item
     // Blink's FlexibleBoxAlgorithm expects it to be content + scrollbar widths,
     // but no padding or border.
-    // The ClampNegativeToZero is needed for the last canvas element in
-    // flexbox-flex-basis-content-001a.html. It's possibly only needed because
-    // we don't properly account for borders+padding when multiplying by the
-    // aspect ratio.
+    DCHECK_GE(flex_base_border_box, main_axis_border_padding);
     LayoutUnit flex_base_content_size =
-        (flex_base_border_box - main_axis_border_padding).ClampNegativeToZero();
+        flex_base_border_box - main_axis_border_padding;
 
     const Length& min = is_horizontal_flow_ ? child.Style().MinWidth()
                                             : child.Style().MinHeight();