Text selection is updated by timer when webview's geometry is changed.

Text selection is updated by timer when webview's geometry is changed.

[Title] Text selection is updated by timer when webview's geometry is changed.
[Issue#] P130730-4510
[Problem] When text selection is working and screen is rotated in Email composer,
          webview is not scrolled smoothly.
[Cause] Whenever webview is scrolled, webview's geometry is changed.
        And whenever webview's geometry is changed, text selection is updated immediately.
        So, function to show context menu is called too many during scrolling.
[Solution] Added timer for updating text selection.
           - When webview's geometry is chagned, text selection is updated by timer.

Change-Id: I1cfdcf0145d6de08a95a0d17535dc50210faad95