WebGL: fix DrawingBuffer::m_colorBuffer texture leak.

After r164809, DrawingBuffer::m_colorBuffer leaks in the destructor.

In addition, this CL changes the order of deletion; fbo first, texture second.
It's because fbo depends on texture.

BUG=331181, 349418

Review URL: https://codereview.chromium.org/187293004

git-svn-id: svn://svn.chromium.org/blink/trunk@168727 bbb929c8-8fbe-4397-9dbb-9b2b20218538