When binding a vertex array object, it was not changing the tracked index
buffer binding.  This was causing the buffer bindings to sometimes not be
updated between index and non-indexed draw calls.

Fixes:
 * Intermittent crashes in chromium startup.
 * conformance/rendering/many-draw-calls.html
 * conformance/rendering/framebuffer-switch.html
 * conformance/attribs/gl-bindAttribLocation-aliasing.html
 * conformance/attribs/gl-vertex-attrib-render.html
 * conformance/buffers/index-validation-verifies-too-many-indices.html

BUG=angleproject:883

Change-Id: I34ed1ebc65b339329c0f9ab9c28a392f552ed3d8
Reviewed-on: https://chromium-review.googlesource.com/273300Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>