WebGL: use storage texture for DrawingBuffer only if CMAA is enabled.

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
index f86a72c3a2e0aa0845052030b27b6a27259b1d2c..5e52cbf43e9de39e0bf2fee0ead29aee6fcfa51f 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -485,7 +485,13 @@ bool DrawingBuffer::initialize(const IntSize& size, bool useMultisampling)
             m_antiAliasingMode = ScreenSpaceAntialiasing;
         }
     }
-    m_storageTextureSupported = m_webGLVersion > WebGL1 || m_extensionsUtil->supportsExtension("GL_EXT_texture_storage");
+    // TODO(dshwang): enable storage texture on all platform. crbug.com/557848
+    // Linux ATI bot fails WebglConformance.conformance_textures_misc_tex_image_webgl
+    // So use storage texture only if ScreenSpaceAntialiasing is enabled,
+    // because ScreenSpaceAntialiasing is much faster with storage texture.
+    m_storageTextureSupported = (m_webGLVersion > WebGL1
+        ||  m_extensionsUtil->supportsExtension("GL_EXT_texture_storage"))
+        && m_antiAliasingMode == ScreenSpaceAntialiasing;
     m_sampleCount = std::min(4, maxSampleCount);
 
     m_gl->GenFramebuffers(1, &m_fbo);