Reland "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 e0a19da5761fbfe3445f69b93b6f22155621eb72..7ee4480ecd9c588b7ea96d345d49ab1661b94494 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
@@ -253,6 +253,7 @@ private:
         DISALLOW_NEW();
         GLuint textureId = 0;
         GLuint imageId = 0;
+        bool immutable = false;
 
         // A GpuMemoryBuffer is a concept that the compositor understands. and
         // is able to operate on. The id is scoped to renderer process.
@@ -321,9 +322,10 @@ 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);
+    // Create a storage texture if possible.
+    // Return true if a storage texture is created.
+    // Return false if a regular texture is created.
+    bool createStorageTextureIfPossible(GLenum target, GLenum internalformat, GLsizei width, GLsizei height, GLint border, GLenum format, GLenum type);
     // Allocate buffer storage to be sent to compositor using either texImage2D or CHROMIUM_image based on available support.
     void deleteChromiumImageForTexture(TextureInfo*);
 
@@ -376,6 +378,7 @@ private:
     const bool m_wantAlphaChannel;
     const bool m_premultipliedAlpha;
     bool m_hasImplicitStencilBuffer = false;
+    bool m_storageTextureSupported = false;
     struct FrontBufferInfo {
         TextureInfo texInfo;
         WebExternalTextureMailbox mailbox;