This patch intends to clean up all coding style issues in
InstancingTest.cpp.
1. Replace SHADER_SOURCE by std::string
2. Use (expected, actual) as the order of the parameters
   in ASSERT_NE
3. Replace C-style type cast operators into C++ ones.
4. Use ANGLE_GL_PROGRAM to build shader programs
5. Use constexpr when necessary
6. Other format changes done by 'git cl format'

BUG=angleproject:2005

Change-Id: I37d866c5df681160a66226bf5927dd6e00a6d6b2
Reviewed-on: https://chromium-review.googlesource.com/528649
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>