[Backport] Make previousLinePosition() not to use dangling RootInlineBox

This patch makes |previousLinePosition()| not to use dangling |RootInlineBox|
pointer to avoid use-after-free.

Before this patch, |isEditablePosition()| is called with |DoUpdateStyle|
parameter to update layout tree if needed. Usually, layout tree isn't updated
by this |isEditablePosition()| call since |previousLinePosition()| updates
layout tree at entry. However, if there are pending style sheet, e.g. @import
directive, and HTML import, e.g link rel=import, layout tree is updated since
document isn't rendering ready, |haveImportLoaded()| &&
|haveRenderBlockingStyleSheetsLoaded()|.

BUG=618237
TEST=LayoutTests/editing/selection/modify_move/move_backward_line_import_crash.html

Committed: https://crrev.com/fb81c66590538c2487a34b8623066a22d0b27dff
Review-Url: https://codereview.chromium.org/2082893005
Cr-Original-Commit-Position: refs/heads/master@{#401231}
Cr-Commit-Position: refs/heads/master@{#401581}
(cherry picked from commit e9c943f368d15bbfe414aedf5e001792257f3eeb)

Review URL: https://codereview.chromium.org/2149913003 .
(CVE-2016-5127)

Change-Id: Id64821526b26d8268ecd2585279cbec6f2c5a1c5
Reviewed-by: Michael Brüning <michael.bruning@qt.io>

1 files changed, 1 insertions, 1 deletions

diff --git a/chromium/third_party/WebKit/Source/core/editing/VisibleUnits.cpp b/chromium/third_party/WebKit/Source/core/editing/VisibleUnits.cpp
index 4dc8164f256..cc442011d10 100644
--- a/chromium/third_party/WebKit/Source/core/editing/VisibleUnits.cpp
+++ b/chromium/third_party/WebKit/Source/core/editing/VisibleUnits.cpp
@@ -997,7 +997,7 @@ VisiblePosition previousLinePosition(const VisiblePosition &visiblePosition, Lay
     if (root) {
         // FIXME: Can be wrong for multi-column layout and with transforms.
         LayoutPoint pointInLine = absoluteLineDirectionPointToLocalPointInBlock(root, lineDirectionPoint);
-        LayoutObject& layoutObject = root->closestLeafChildForPoint(pointInLine, isEditablePosition(p))->layoutObject();
+        LayoutObject& layoutObject = root->closestLeafChildForPoint(pointInLine, isEditablePosition(p, ContentIsEditable, DoNotUpdateStyle))->layoutObject();
         Node* node = layoutObject.node();
         if (node && editingIgnoresContent(node))
             return VisiblePosition(positionInParentBeforeNode(*node));