Use a gradient drawable for the tab window background

Fixes banding and scaling issues caused by the image drawable.

Change-Id: Ib3ff903c0d9d97a04b9a4f92de42ba69d6e47a0a
Fix the problem that the layout of the suggestion drop down list does not fit to the screen width when an orientation changes.

Steps to reproduce:
1. Set the orientation of the Browser to landscape mode.
2. Type some keys in the url input view to show some suggestions.
3. Set the orientation to portrait mode again.

What heppended:
The width of the suggestion layout exceeds the screen width.
The arrow icon on the right side couldn't be seen.

Additional comments:
On some devices, setupDropDown() doesn't work correctly.
On the Samsung Galaxy S3, for instance, setupDropDown() produces left-hand margins.

The simplest way to fix these problems is to call setDropDownAnchor() when the url input view is created.

Change-Id: Icfe979033da235a960b8b306f291802cb7c46646
Signed-off-by: Sungmann Cho <sungmann.cho@navercorp.com>