Implement sub source for WebGL2's clearBuffer*().

third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.idl

Index: third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.idl
diff --git a/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.idl b/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.idl
index c645b8796134414c0b7ecf4c4e0d9ad3fcc2b02e..75fe60beb9ff13ddf3861f715d00e317a204f8e1 100644
--- a/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.idl
+++ b/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.idl
@@ -465,12 +465,12 @@ typedef unsigned long long GLuint64;
 
     /* Multiple Render Targets */
     void drawBuffers(sequence<GLenum> buffers);
-    void clearBufferiv(GLenum buffer, GLint drawbuffer, Int32Array value);
-    void clearBufferiv(GLenum buffer, GLint drawbuffer, sequence<GLint> value);
-    void clearBufferuiv(GLenum buffer, GLint drawbuffer, Uint32Array value);
-    void clearBufferuiv(GLenum buffer, GLint drawbuffer, sequence<GLuint> value);
-    void clearBufferfv(GLenum buffer, GLint drawbuffer, Float32Array value);
-    void clearBufferfv(GLenum buffer, GLint drawbuffer, sequence<GLfloat> value);
+    void clearBufferiv(GLenum buffer, GLint drawbuffer, Int32Array value, optional GLuint srcOffset = 0);
+    void clearBufferiv(GLenum buffer, GLint drawbuffer, sequence<GLint> value, optional GLuint srcOffset = 0);
+    void clearBufferuiv(GLenum buffer, GLint drawbuffer, Uint32Array value, optional GLuint srcOffset = 0);
+    void clearBufferuiv(GLenum buffer, GLint drawbuffer, sequence<GLuint> value, optional GLuint srcOffset = 0);
+    void clearBufferfv(GLenum buffer, GLint drawbuffer, Float32Array value, optional GLuint srcOffset = 0);
+    void clearBufferfv(GLenum buffer, GLint drawbuffer, sequence<GLfloat> value, optional GLuint srcOffset = 0);
     void clearBufferfi(GLenum buffer, GLint drawbuffer, GLfloat depth, GLint stencil);
 
     /* Query Objects */