Bug 1501665 Part 6: Fix an edge case in Document::GetViewportInfo to return css scaled sizes. r=botond

This fixes a problem in an edge case that we are evidently not testing.
If width is 0 and height is auto, we are currently setting height to
the unscaled (not CSS units) display height value.

In addition to this fix, this patch does two drive-by cleanups of code
that uses "defaultScale":

    The redundantly-defined defaultPixelScale is removed.
    The masking block-level definition of defaultScale is renamed to bestFitScale.

Differential Revision: https://phabricator.services.mozilla.com/D16774

--HG--
extra : source : 9eebe767ef20b9ec635d90fc9844cf7dbe3403de

dom/base/Document.cpp

@@ -6772,10 +6772,9 @@ nsViewportInfo Document::GetViewportInfo(const ScreenIntSize& aDisplaySize) {
       CSSCoord maxHeight = mMaxHeight;
 
       // aDisplaySize is in screen pixels; convert them to CSS pixels for the
-      // viewport size.
-      CSSToScreenScale defaultPixelScale =
-          layoutDeviceScale * LayoutDeviceToScreenScale(1.0f);
-      CSSSize displaySize = ScreenSize(aDisplaySize) / defaultPixelScale;
+      // viewport size. We need to use this scaled size for any clamping of
+      // width or height.
+      CSSSize displaySize = ScreenSize(aDisplaySize) / defaultScale;
 
       // Resolve device-width and device-height first.
       if (maxWidth == nsViewportInfo::DeviceSize) {
@@ -6852,7 +6851,7 @@ nsViewportInfo Document::GetViewportInfo(const ScreenIntSize& aDisplaySize) {
       // https://drafts.csswg.org/css-device-adapt/#resolve-height
       if (height == nsViewportInfo::Auto) {
         if (aDisplaySize.width == 0) {
-          height = aDisplaySize.height;
+          height = displaySize.height;
         } else {
           height = width * aDisplaySize.height / aDisplaySize.width;
         }
@@ -6895,8 +6894,8 @@ nsViewportInfo Document::GetViewportInfo(const ScreenIntSize& aDisplaySize) {
       // Also recalculate the default zoom, if it wasn't specified in the
       // metadata, and the width is specified.
       if (mScaleStrEmpty && !mWidthStrEmpty) {
-        CSSToScreenScale defaultScale(float(aDisplaySize.width) / size.width);
-        scaleFloat = (scaleFloat > defaultScale) ? scaleFloat : defaultScale;
+        CSSToScreenScale bestFitScale(float(aDisplaySize.width) / size.width);
+        scaleFloat = (scaleFloat > bestFitScale) ? scaleFloat : bestFitScale;
       }
 
       size.height = clamped(size.height, effectiveMinSize.height,