Make RemoveFormat command to work with mixed editable

Source/core/editing/ApplyStyleCommand.cpp

Index: Source/core/editing/ApplyStyleCommand.cpp
diff --git a/Source/core/editing/ApplyStyleCommand.cpp b/Source/core/editing/ApplyStyleCommand.cpp
index a5a5df35facd8abe3c7bc2829451f647d519217d..88cb5463c84f391daa9118b7de6ce80be1d1497d 100644
--- a/Source/core/editing/ApplyStyleCommand.cpp
+++ b/Source/core/editing/ApplyStyleCommand.cpp
@@ -1155,7 +1155,7 @@ void ApplyStyleCommand::removeInlineStyle(EditingStyle* style, const Position &s
                 if (s.deprecatedNode() == elem) {
                     // Since elem must have been fully selected, and it is at the start
                     // of the selection, it is clear we can set the new s offset to 0.
-                    ASSERT(s.anchorType() == Position::PositionIsBeforeAnchor || s.offsetInContainerNode() <= 0);
+                    ASSERT(s.anchorType() == Position::PositionIsBeforeAnchor || s.anchorType() == Position::PositionIsBeforeChildren || s.offsetInContainerNode() <= 0);

[Yuta Kitamura, 2014/07/01 09:12:35]

Who creates the position of type PositionIsBeforeChildren? It's possible
that the creator of the position is to blame, instead of this assertion.

[yosin_UTC9, 2014/07/02 03:30:36]

In the attached test case, |firstPositionInOrBeforeNode(next.get())|, L1159=just
below assertion, returns 
|PositionIsBeforeChildren| for B element.

Here is definition of |firstPositionInOrBeforeNode|
inline Position firstPositionInOrBeforeNode(Node* node)
{
    if (!node)
        return Position();
    return editingIgnoresContent(node) ? positionBeforeNode(node) :
firstPositionInNode(node);
}

[Yuta Kitamura, 2014/07/02 07:27:21]

I don't understand your reasoning. This ASSERT is located
before the assignment at line 1159. How does that affect
the value of |s.anchorType()| at line 1158?

[yosin_UTC9, 2014/07/02 08:04:03]

The ASSERT and firstPositionInOrBeforeNode() is in while loop. |s| is updated
here and checked in next iteration.

[Yuta Kitamura, 2014/07/03 05:11:29]

Okay, I think this is an oversight from rniwa's change
<https://chromium.googlesource.com/chromium/blink/+/0db51d7c3a2245cb7b99c6956f...>.

                     s = firstPositionInOrBeforeNode(next.get());
                 }
                 if (e.deprecatedNode() == elem) {