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When we run content_shell in the single process mode with tests using the clipboard, the renderer thread passes WebString to the browser thread. It causes leak.
To avoid this, change the member WebString to NullableString16.

BUG=328158
TEST=content_shell --dump-render-tree --single-process ../../third_party/WebKit/LayoutTests/editing/pasteboard/createMarkup-assert.xml

Review URL: https://codereview.chromium.org/135853010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247850 0039d316-1c4b-4281-b951-d872f2087c98