Add some string helper classes.

*re-land with build fix for Clang*

These helper functions mirror some utilities in Chrome, for splitting
strings along whitespace, or reading file contents into a string.

Also remove the hack for skipping the doubly-defined GLX header.

BUG=angleproject:892,angleproject:998

Change-Id: Ife43fbf5035a3be7820460bea1b26d0e632a4fb0
Reviewed-on: https://chromium-review.googlesource.com/272518
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/libANGLE/renderer/gl/FunctionsGL.cpp b/src/libANGLE/renderer/gl/FunctionsGL.cpp
index e93a8c7..536572e 100644
--- a/src/libANGLE/renderer/gl/FunctionsGL.cpp
+++ b/src/libANGLE/renderer/gl/FunctionsGL.cpp
@@ -10,6 +10,7 @@
 
 #include <algorithm>
 
+#include "common/string_utils.h"
 #include "libANGLE/renderer/gl/renderergl_utils.h"
 
 namespace rx
@@ -779,7 +780,7 @@
     else
     {
         const char *exts = reinterpret_cast<const char*>(getString(GL_EXTENSIONS));
-        extensions = TokenizeExtensionsString(exts);
+        angle::SplitStringAlongWhitespace(std::string(exts), &extensions);
     }
 
     // 1.0