Bug 1253840 - patch 1 - Allow text-align:justify to take effect when using white-space:pre-wrap. r=emilio

The CSS Text 3 spec allows this (CSS2 didn't), and other browsers
support it, so for better webcompat/interoperability, we should do the
same. Tests will need updating accordingly.

This first patch enables the property to take effect, but the resulting
layout is not yet correct because the (non-collapsed) whitespace at line
wrap positions needs to be forcibly "hung" into the margin for correct
justification; see following patch.

Differential Revision: https://phabricator.services.mozilla.com/D178210

layout/generic/nsTextFrame.cpp

@@ -2229,8 +2229,7 @@ already_AddRefed<gfxTextRun> BuildTextRunsScanner::BuildTextRunForFrames(
     flags |= GetSpacingFlags(f);
     nsTextFrameUtils::CompressionMode compression =
         GetCSSWhitespaceToCompressionMode(f, textStyle);
-    if ((enabledJustification || f->ShouldSuppressLineBreak()) &&
-        !textStyle->WhiteSpaceIsSignificant() && !isSVG) {
+    if ((enabledJustification || f->ShouldSuppressLineBreak()) && !isSVG) {
       flags |= gfx::ShapedTextFlags::TEXT_ENABLE_SPACING;
     }
     fontStyle = f->StyleFont();
@@ -9499,8 +9498,7 @@ void nsTextFrame::ReflowText(nsLineLayout& aLineLayout, nscoord aAvailableWidth,
   }
 
   // Compute space and letter counts for justification, if required
-  if (!textStyle->WhiteSpaceIsSignificant() &&
-      (lineContainer->StyleText()->mTextAlign == StyleTextAlign::Justify ||
+  if ((lineContainer->StyleText()->mTextAlign == StyleTextAlign::Justify ||
        lineContainer->StyleText()->mTextAlignLast ==
            StyleTextAlignLast::Justify ||
        shouldSuppressLineBreak) &&