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/FramebufferRenderMipmapTest.cpp b/src/tests/gl_tests/FramebufferRenderMipmapTest.cpp
index 660b6d6..d46c736 100644
--- a/src/tests/gl_tests/FramebufferRenderMipmapTest.cpp
+++ b/src/tests/gl_tests/FramebufferRenderMipmapTest.cpp
@@ -25,23 +25,19 @@
     {
         ANGLETest::SetUp();
 
-        const std::string vsSource = SHADER_SOURCE
-        (
-            attribute highp vec4 position;
+        const std::string vsSource =
+            R"(attribute highp vec4 position;
             void main(void)
             {
                 gl_Position = position;
-            }
-        );
+            })";
 
-        const std::string fsSource = SHADER_SOURCE
-        (
-            uniform highp vec4 color;
+        const std::string fsSource =
+            R"(uniform highp vec4 color;
             void main(void)
             {
                 gl_FragColor = color;
-            }
-        );
+            })";
 
         mProgram = CompileProgram(vsSource, fsSource);
         if (mProgram == 0)