Save previous paint offset in LayoutObject

LayoutTests/paint/invalidation/spv2/multicol-as-paint-container.html

Index: LayoutTests/paint/invalidation/spv2/multicol-as-paint-container.html
diff --git a/LayoutTests/paint/invalidation/spv2/multicol-as-paint-container.html b/LayoutTests/paint/invalidation/spv2/multicol-as-paint-container.html
index 07848e3eedb9daad80e0420d94b667a7568e4583..3854fd1c1451bb42be4f1bf60e30fb37e1862de5 100644
--- a/LayoutTests/paint/invalidation/spv2/multicol-as-paint-container.html
+++ b/LayoutTests/paint/invalidation/spv2/multicol-as-paint-container.html
@@ -23,6 +23,8 @@
         "InlineTextBox 'eget, dapibus'",
         "InlineTextBox 'ultrices dolor.'",
         "LayoutMultiColumnSet (anonymous)",
+        "InlineTextBox 'Lorem ipsum'",

[pdr., 2015/09/22 22:44:44]

I wouldn't have expected diffs when spv2 is enabled... doesn't this mean a real
behavior change has been introduced?

[Xianzhu, 2015/09/23 18:43:18]

Noticed a problem of paint offset caching of line boxes: we don't have separate
paint offset cache for each line box. Previous update of paint offset affects
later line boxes.

Reverted https://codereview.chromium.org/1340133003/ (paint offset for
Inlineboxes). Now check paint offset change in BlockPainter and InlinePainter
and invalidate descendant lineboxes when paint offset changes.

+        "InlineTextBox 'ultricies arcu,'",
     ];
     function paintInvalidationTest() {
         var target = document.getElementById('target');