webgl: use immutable texture for the default FBO.

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

Index: third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
diff --git a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
index 0b4a0e37b863b4acdbcd5b5c6dc8363b581cac4d..b467867abb90574bb480bcd106c080eb06454619 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
@@ -69,10 +69,12 @@ class PLATFORM_EXPORT DrawingBuffer : public RefCounted<DrawingBuffer>, public W
     struct TextureInfo {
         Platform3DObject textureId;
         WGC3Duint imageId;
+        bool immutable;
 
         TextureInfo()
             : textureId(0)
             , imageId(0)
+            , immutable(false)
         {
         }
     };
@@ -230,9 +232,6 @@ private:
     // Helper function to flip a bitmap vertically.
     void flipVertically(uint8_t* data, int width, int height);
 
-    // Helper to texImage2D with pixel==0 case: pixels are initialized to 0.
-    // By default, alignment is 4, the OpenGL default setting.
-    void texImage2DResourceSafe(GLenum target, GLint level, GLenum internalformat, GLsizei width, GLsizei height, GLint border, GLenum format, GLenum type, GLint alignment = 4);
     // Allocate buffer storage to be sent to compositor using either texImage2D or CHROMIUM_image based on available support.
     void allocateTextureMemory(TextureInfo*, const IntSize&);
     void deleteChromiumImageForTexture(TextureInfo*);
@@ -251,6 +250,7 @@ private:
     bool m_multisampleExtensionSupported;
     bool m_packedDepthStencilExtensionSupported;
     bool m_discardFramebufferSupported;
+    bool m_storageTextureSupported;
     Platform3DObject m_fbo;
     // DrawingBuffer's output is double-buffered. m_colorBuffer is the back buffer.
     TextureInfo m_colorBuffer;