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/IndexBufferOffsetTest.cpp b/src/tests/gl_tests/IndexBufferOffsetTest.cpp
index 329848a..d344c17 100644
--- a/src/tests/gl_tests/IndexBufferOffsetTest.cpp
+++ b/src/tests/gl_tests/IndexBufferOffsetTest.cpp
@@ -29,14 +29,22 @@
         ANGLETest::SetUp();
 
         const std::string vertexShaderSource =
-            SHADER_SOURCE(precision highp float; attribute vec2 position;
+            R"(precision highp float;
+            attribute vec2 position;
 
-                          void main() { gl_Position = vec4(position, 0.0, 1.0); });
+            void main()
+            {
+                gl_Position = vec4(position, 0.0, 1.0);
+            })";
 
         const std::string fragmentShaderSource =
-            SHADER_SOURCE(precision highp float; uniform vec4 color;
+            R"(precision highp float;
+            uniform vec4 color;
 
-                          void main() { gl_FragColor = color; });
+            void main()
+            {
+                gl_FragColor = color;
+            })";
 
         mProgram = CompileProgram(vertexShaderSource, fragmentShaderSource);
         ASSERT_NE(0u, mProgram);