[Minor Cleanup] Remove unnecessary constructor from WebCompositionUnderline.h
[Minor Cleanup] Remove unnecessary constructor from WebCompositionUnderline.h

This constructor has no references to it. Hence, removing it.

BUG=None

Review URL: https://codereview.chromium.org/426573004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@179083 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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