Use C++11 raw string literals instead of SHADER_SOURCE macro

This is better in many ways:
1. It doesn't confuse clang format
2. \n doesn't need to be included after preprocessor directives like
   the version directive.
3. It's using built-in functionality instead of something custom.

Raw string literals should be the preferred way to include shader
source in C++ files going forward.

BUG=angleproject:2157
TEST=angle_end2end_tests

Change-Id: I8b236a6e2d5c25d920297e5bc5b5b143eddeba1f
Reviewed-on: https://chromium-review.googlesource.com/671046
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
diff --git a/src/tests/gl_tests/PBOExtensionTest.cpp b/src/tests/gl_tests/PBOExtensionTest.cpp
index bc6179a..baa63a4 100644
--- a/src/tests/gl_tests/PBOExtensionTest.cpp
+++ b/src/tests/gl_tests/PBOExtensionTest.cpp
@@ -34,18 +34,24 @@
             glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
 
             const char *vertexShaderSrc =
-                SHADER_SOURCE(attribute vec4 aTest; attribute vec2 aPosition; varying vec4 vTest;
+                R"(attribute vec4 aTest;
+                attribute vec2 aPosition;
+                varying vec4 vTest;
 
-                              void main() {
-                                  vTest        = aTest;
-                                  gl_Position  = vec4(aPosition, 0.0, 1.0);
-                                  gl_PointSize = 1.0;
-                              });
+                void main()
+                {
+                    vTest        = aTest;
+                    gl_Position  = vec4(aPosition, 0.0, 1.0);
+                    gl_PointSize = 1.0;
+                })";
 
             const char *fragmentShaderSrc =
-                SHADER_SOURCE(precision mediump float; varying vec4 vTest;
-
-                              void main() { gl_FragColor = vTest; });
+                R"(precision mediump float;
+                varying vec4 vTest;
+                void main()
+                {
+                    gl_FragColor = vTest;
+                })";
 
             mProgram = CompileProgram(vertexShaderSrc, fragmentShaderSrc);