Protect an element while removing an attribute.

It's possible for attribute removal to do non-trivial work (e.g., in the case
of removing an iframe's src attr, which navigates the iframe to about:blank)

BUG=382279
TEST=http/tests/misc/adopt-iframe-src-attr-after-remove.html

Review URL: https://codereview.chromium.org/341273006

git-svn-id: svn://svn.chromium.org/blink/trunk@176668 bbb929c8-8fbe-4397-9dbb-9b2b20218538