Bug 1174003 part 2: [css-flexbox] Reformat code around GetMinimumWidgetSize call slightly. r=mats

This patch doesn't change behavior. It's purely to allow the next patch to be more surgical. Specifically, this patch:
 - splits a subtract-and-clamp operation into two separate operations.
 - splits one a comment into two.
...so that the next patch can swap out these variables for new ones, without
pushing these lines over 80 characters.

MozReview-Commit-ID: 4N5sI755CqF

--HG--
extra : rebase_source : 96af106d739256a6871e75d0d34e9691cfbfb0c2

layout/generic/nsFlexContainerFrame.cpp

@@ -1282,13 +1282,14 @@ nsFlexContainerFrame::GenerateFlexItemForChild(
       aPresContext->DevPixelsToAppUnits(
         aAxisTracker.GetCrossComponent(widgetMinSize));
 
-    // GMWS() returns border-box. We need content-box, so subtract
-    // borderPadding (but don't let that push our min sizes below 0).
+    // GetMinimumWidgetSize() returns border-box. We need content-box, so
+    // subtract borderPadding.
     nsMargin& bp = childRI.ComputedPhysicalBorderPadding();
-    widgetMainMinSize = std::max(widgetMainMinSize -
-                                 aAxisTracker.GetMarginSizeInMainAxis(bp), 0);
-    widgetCrossMinSize = std::max(widgetCrossMinSize -
-                                  aAxisTracker.GetMarginSizeInCrossAxis(bp), 0);
+    widgetMainMinSize -= aAxisTracker.GetMarginSizeInMainAxis(bp);
+    widgetCrossMinSize -= aAxisTracker.GetMarginSizeInCrossAxis(bp);
+    // ... (but don't let that push these min sizes below 0).
+    widgetMainMinSize = std::max(0, widgetMainMinSize);
+    widgetCrossMinSize = std::max(0, widgetCrossMinSize);
 
     if (!canOverride) {
       // Fixed-size widget: freeze our main-size at the widget's mandated size.