WebGL: Fix assertion hit of gpu tests.

This CL removes the invalid assertion in DrawingBuffer::prepareMailbox(); ASSERT(!m_destructionInProgress).
It can be hit in the following sequence.
1. WebGL draws something.
2. The compositor begins the frame.
3. Javascript makes a context lost using WEBGL_lose_context extension.
4. Assertion hits.

TEST=ContextLost.WebGLContextLostFromGPUProcessExit
BUG=365904

Review URL: https://codereview.chromium.org/259533003

git-svn-id: svn://svn.chromium.org/blink/trunk@172564 bbb929c8-8fbe-4397-9dbb-9b2b20218538