Rely on GPU command buffer to validate WebGL vertex attrib state

This removes some validation that is already done by the GPU command
buffer, reducing complexity and overhead.

There were also some deficiencies in the removed validation:
m_onePlusMaxEnabledAttribIndex was not being updated when VAOs were
changed, and vertex attrib divisor state was not checked for
non-instanced draw calls, even though it affects them as well.

BUG=390827
TEST=WebGL conformance tests

Review URL: https://codereview.chromium.org/444333003

git-svn-id: svn://svn.chromium.org/blink/trunk@179810 bbb929c8-8fbe-4397-9dbb-9b2b20218538