Remove usage of std::log2 in tests.

For some reason, this was giving a link error on Android.
Change it to use the mathutil version to be sure.

BUG=angleproject:1493
BUG=chromium:638323

Change-Id: I454ba4609d7818f50c668bb7c1a3b684f9ca8444
Reviewed-on: https://chromium-review.googlesource.com/381671
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/src/tests/gl_tests/TextureTest.cpp b/src/tests/gl_tests/TextureTest.cpp
index 2595359..8be8016 100644
--- a/src/tests/gl_tests/TextureTest.cpp
+++ b/src/tests/gl_tests/TextureTest.cpp
@@ -4,8 +4,7 @@
 // found in the LICENSE file.
 //
 
-#include <cmath>
-
+#include "common/mathutil.h"
 #include "test_utils/ANGLETest.h"
 #include "test_utils/gl_raii.h"
 
@@ -3494,7 +3493,7 @@
     const int size = getWindowWidth();
 
     auto dim   = [size](int level) { return size >> level; };
-    int levels = static_cast<int>(std::log2(size));
+    int levels = gl::log2(size);
 
     glActiveTexture(GL_TEXTURE0);
     glBindTexture(GL_TEXTURE_2D, mTexture2D);