[css-flex] Update to newer spec about definite flex item sizes

Implements this change:
https://github.com/w3c/csswg-drafts/commit/5b5db39d21f3658ae2f4d7992daaf822aca178d8

external/wpt/css/css-flexbox/percentage-heights-003.html ostensibly tests
this, but I don't think the test is correct (and we don't pass it)

TESTED=css3/flexbox/definite-main-size.html

Bug: 784059
Change-Id: I8ee0ee797b54a8166849ab6e9b9f019b9e43760b
Reviewed-on: https://chromium-review.googlesource.com/1240871
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593707}

third_party/WebKit/LayoutTests/css3/flexbox/definite-main-size.html

@@ -61,10 +61,10 @@ rules and resolve the percentage:</p>
   <div class="rect flex-none"></div>
 </div>
 
-<p>auto flex-basis, we should ignore the percentage height here:</p>
+<p>auto flex-basis. This is still definite.</p>
 <div class="flexbox column" style="height: 300px;">
   <div data-expected-height="50">
-    <div style="height: 50%;" data-expected-height="50">
+    <div style="height: 50%;" data-expected-height="25">
       <div class="rect"></div>
     </div>
   </div>

third_party/blink/renderer/core/layout/layout_flexible_box.cc

@@ -1073,18 +1073,10 @@ bool LayoutFlexibleBox::MainSizeIsDefiniteForPercentageResolution(
     const LayoutBox& child) const {
   // This function implements section 9.8. Definite and Indefinite Sizes, case
   // 2) of the flexbox spec.
-  // We need to check for the flexbox to have a definite main size, and for the
-  // flex item to have a definite flex basis.
-  const Length& flex_basis = FlexBasisForChild(child);
-  if (!MainAxisLengthIsDefinite(child, flex_basis))
+  // We need to check for the flexbox to have a definite main size.
+  // We make up a percentage to check whether we have a definite size.
+  if (!MainAxisLengthIsDefinite(child, Length(0, kPercent)))
     return false;
-  if (!flex_basis.IsPercentOrCalc()) {
-    // If flex basis had a percentage, our size is guaranteed to be definite or
-    // the flex item's size could not be definite. Otherwise, we make up a
-    // percentage to check whether we have a definite size.
-    if (!MainAxisLengthIsDefinite(child, Length(0, kPercent)))
-      return false;
-  }
 
   if (HasOrthogonalFlow(child))
     return child.HasOverrideLogicalHeight();