Bug 1731541 - Implement text-wrap: balance for nsBlockFrame reflow. r=emilio

A simple form of balance for short blocks, implemented by incrementally reducing
the effective inline-size used during line-breaking, up to the point where an
extra line would be created.

This fails the test text-wrap-balance-line-clamp-001.html, but it's unclear to me
if that test is correct (see w3c/csswg-drafts#9310).
If we do want the behavior expected by that test, an additional patch to handle
the interaction with line-clamp will be required.

Depends on D187543

Differential Revision: https://phabricator.services.mozilla.com/D187544

UltraBlame original commit: 0fd4431f527935234d764a167cc5a20acfc323eb

Author: marco-c

layout/generic/BlockReflowState.cpp

@@ -265,6 +265,9 @@ aConsumedBSize
 const
 nscoord
 aEffectiveContentBoxBSize
+const
+nscoord
+aInset
 )
 :
 mBlock
@@ -287,6 +290,10 @@ GetWritingMode
 (
 )
 )
+mInsetForBalance
+(
+aInset
+)
 mPushedFloats
 (
 nullptr

layout/generic/BlockReflowState.h

@@ -776,6 +776,11 @@ aConsumedBSize
 const
 nscoord
 aEffectiveContentBoxBSize
+const
+nscoord
+aInset
+=
+0
 )
 ;
 /
@@ -1797,10 +1802,12 @@ this
 object
 nsBlockFrame
 *
+const
 mBlock
 ;
 nsPresContext
 *
+const
 mPresContext
 ;
 const
@@ -2231,6 +2238,42 @@ wm
 }
 /
 /
+Amount
+of
+inset
+to
+apply
+during
+line
+-
+breaking
+used
+by
+text
+-
+wrap
+:
+balance
+/
+/
+to
+adjust
+line
+-
+breaks
+for
+more
+consistent
+lengths
+throughout
+the
+block
+.
+nscoord
+mInsetForBalance
+;
+/
+/
 Physical
 size
 .
@@ -2646,6 +2689,7 @@ nsIFrame
 PreReflowBlockLevelLogicalSkipSides
 )
 .
+const
 LogicalMargin
 mBorderPadding
 ;
@@ -2990,6 +3034,7 @@ nsIFrame
 >
 mNoWrapFloats
 ;
+const
 nscoord
 mMinLineHeight
 ;