libstdc++ usage of std::floorf in NG out of flow layout part breaks libstdc++ build.

Use math.h floorf instead of std::floorf, as latter is not available in libstdc++
library.

Fixes this build error:
../../third_party/blink/renderer/core/layout/ng/ng_out_of_flow_layout_part.cc: In member function ‘void blink::NGOutOfFlowLayoutPart::ComputeStartFragmentIndexAndRelativeOffset(const blink::NGOutOfFlowLayoutPart::ContainingBlockInfo&, blink::WritingMode, WTF::wtf_size_t*,
 blink::LogicalOffset*) const’:
../../third_party/blink/renderer/core/layout/ng/ng_out_of_flow_layout_part.cc:1194:16: error: ‘floorf’ is not a member of ‘std’; did you mean ‘floor’?
 1194 |       int(std::floorf(remaining_block_offset / fragmentainer_block_size));
      |                ^~~~~~
      |                floor

Bug: 957519
Change-Id: I7d8f78c04948a6c0e37cbca59bdbca485fc839d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2394165
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804612}

third_party/blink/renderer/core/layout/ng/ng_out_of_flow_layout_part.cc

@@ -4,6 +4,8 @@
 
 #include "third_party/blink/renderer/core/layout/ng/ng_out_of_flow_layout_part.h"
 
+#include <math.h>
+
 #include "third_party/blink/renderer/core/layout/layout_block.h"
 #include "third_party/blink/renderer/core/layout/layout_box.h"
 #include "third_party/blink/renderer/core/layout/layout_flexible_box.h"
@@ -1234,7 +1236,7 @@ void NGOutOfFlowLayoutPart::ComputeStartFragmentIndexAndRelativeOffset(
   }
 
   wtf_size_t additional_fragment_count =
-      int(std::floorf(remaining_block_offset / fragmentainer_block_size));
+      int(floorf(remaining_block_offset / fragmentainer_block_size));
   *start_index = child_index + additional_fragment_count;
   offset->block_offset = remaining_block_offset -
                          additional_fragment_count * fragmentainer_block_size;