The patch adds two tests to measure the performance of the
ANGLE_multiview extension implementation:
1) The first test renders onto two views by issuing thousands of state
changes and draw calls. The aim of the test is to stress the CPU.
2) The second test renders onto two views by drawing with one draw call
half a million quads with multiple attributes per vertex. The attributes
are passed to the fragment shader where they are used for computing the
color. The aim of the test is to stress the GPU's memory system.

The patch also extends the ANGLEPerfTest's functionality to only run
the benchmark if the necessary extensions are available.

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: Ic63d54a69fdddb72439eeeb1951a500fb1247e95
Reviewed-on: https://chromium-review.googlesource.com/597630Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>