Fix n^2 in handleSubtreeModifications.
Fix n^2 in handleSubtreeModifications.

This function was traversing the tree recursively and iteratively at the same
time. Lets just do it recursively instead. This will be somewhat more expensive
for the common case where very few LayoutObjects have the bit set, but it fixes
the n^2 which is good. When I tried to make the traversal be iteratively only
tests failed so there's something more complex to be solved there. Lets fix the
n^2 first and then try to make it be iterative later.

BUG=507498

Review URL: https://codereview.chromium.org/1193323003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@198399 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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