[LayoutNG] Skip "optimistic" float placement if inline-size is known.

If we already know the inline-size of the float before placing it, we
don't need to make a guess at its position before laying it out. We can
just set it right away.

This is just an optimization. No behavior changes intended.

Bug: 915929
Change-Id: I72ce406ef6b32ff5755522e3375dd93c407ee834
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2278050Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786440}

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

@@ -232,19 +232,29 @@ NGPositionedFloat PositionFloat(NGUnpositionedFloat* unpositioned_float,
     // laying it out. This is a float, and in order to calculate its offset, we
     // first need to know its inline-size.
 
-    // TODO(crbug.com/915929): In some cases the inline-size of the float is
-    // already known at this point, and then we should be able to set the
-    // correct layout opportunity right away, i.e. no need for optimistic
-    // placement. However, this only happens in inline formatting contexts, and
-    // we don't support fragmenting floats in inline formatting contexts yet.
-    bool optimistically_placed = true;
-
-    // We'll estimate the offset to be the one we'd get if the float isn't
-    // affected by any other floats in the block formatting context. If this
-    // turns out to be wrong, we'll need to lay out again.
-    LayoutUnit fragmentainer_delta =
-        unpositioned_float->origin_bfc_offset.block_offset +
-        fragment_margins.block_start;
+    LayoutUnit fragmentainer_delta;
+    bool optimistically_placed = false;
+    if (unpositioned_float->layout_result) {
+      // We have already laid out the float to find its inline-size.
+      NGFragment float_fragment(
+          parent_space.GetWritingMode(),
+          unpositioned_float->layout_result->PhysicalFragment());
+      // We can find a layout opportunity and set the fragmentainer offset right
+      // away.
+      opportunity = FindLayoutOpportunityForFloat(
+          *unpositioned_float, *exclusion_space, fragment_margins,
+          float_fragment.InlineSize());
+      fragmentainer_delta = opportunity.rect.start_offset.block_offset +
+                            fragment_margins.block_start;
+    } else {
+      // If we don't know the inline-size yet, we'll estimate the offset to be
+      // the one we'd get if the float isn't affected by any other floats in the
+      // block formatting context. If this turns out to be wrong, we'll need to
+      // lay out again.
+      fragmentainer_delta = unpositioned_float->origin_bfc_offset.block_offset +
+                            fragment_margins.block_start;
+      optimistically_placed = true;
+    }
 
     do {
       NGConstraintSpace space = CreateConstraintSpaceForFloat(