Make PageScaleFactor work for oopif subframes.

third_party/WebKit/Source/core/loader/FrameLoader.cpp

Index: third_party/WebKit/Source/core/loader/FrameLoader.cpp
diff --git a/third_party/WebKit/Source/core/loader/FrameLoader.cpp b/third_party/WebKit/Source/core/loader/FrameLoader.cpp
index 24b4a63d43c5123611eee2045d8400145599a277..d00788375e1777320690be32430200d15521c850 100644
--- a/third_party/WebKit/Source/core/loader/FrameLoader.cpp
+++ b/third_party/WebKit/Source/core/loader/FrameLoader.cpp
@@ -1442,6 +1442,7 @@ void FrameLoader::restoreScrollPositionAndViewState() {
     }
 
     VisualViewport& visualViewport = m_frame->host()->visualViewport();
+    float originalScale = visualViewport.scale();
     if (shouldRestoreScale && shouldRestoreScroll) {
       visualViewport.setScaleAndLocation(m_currentItem->pageScaleFactor(),
                                          FloatPoint(visualViewportOffset));
@@ -1450,6 +1451,9 @@ void FrameLoader::restoreScrollPositionAndViewState() {
     } else if (shouldRestoreScroll) {
       visualViewport.setLocation(FloatPoint(visualViewportOffset));
     }
+    // If necessary, notify client we've restored page scale.
+    if (shouldRestoreScale && (originalScale != visualViewport.scale()))
+      client()->didRestorePageScaleFactorOnLoad(visualViewport.scale());

[kenrb, 2017/01/24 17:13:27]

I am not clear on why this is necessary. Does this code path imply that we have
just completed a page load? Doesn't that mean that previously-existing OOPIFs
will all have been destroyed?

[wjmaclean, 2017/01/24 18:24:26]

Hmm, I don't think so, otherwise one would not expect this to work on pages with
oopif subframes, and it seems to. Wouldn't it make sense that any previously
destroyed subframes would be re-created by the time the page load is completed?

I think any subframes that load later get the PSF from the WebContents as they
are loaded, but I'll have to verify that.

[kenrb, 2017/01/24 18:47:30]

What I meant was, if the frames have been destroyed and recreated, wouldn't they
have had their page scale factors reset anyway, because each would have a newly
initialized compositor?

 
     if (ScrollingCoordinator* scrollingCoordinator =
             m_frame->page()->scrollingCoordinator())