Unbind video texture from mailbox after copying to WebGL.

gpu/GLES2/extensions/CHROMIUM/CHROMIUM_texture_mailbox.txt

 Name
 
     CHROMIUM_texture_mailbox
 
 Name Strings
 
     GL_CHROMIUM_texture_mailbox
 
 Version
 
-    Last Modifed Date: June 2, 2014
+    Last Modifed Date: August 3, 2016
 
 Dependencies
 
     OpenGL ES 2.0 is required.
 
 Overview
 
     This extension defines a way of sharing texture image data between texture
     objects in different contexts where the contexts would not normally share
     texture resources. Five new functions are exported. glGenMailboxCHROMIUM
@@ skipping 40 matching lines @@
 
     INVALID_OPERATION is generated if <mailbox> is invalid.
 
 
     void glProduceTextureDirectCHROMIUM (GLuint texture, GLenum target,
         const GLbyte *mailbox)
 
     Associates the specified texture object with the mailbox name. Performs
     identically to glProduceTextureCHROMIUM except that the texture specified by
     <texture> is used instead of the currently bound texture. This operation
-    does not change the texture bindings or alter the bound texture in any way.
+    does not change the texture bindings or alter the bound texture in any
+    way. If <texture> is 0 then any previous association of the mailbox with a
+    texture object is broken and no new association is created.
 
     <texture> Specifies the name of a texture.
 
 
     void glConsumeTextureCHROMIUM (GLenum target, const GLbyte *mailbox)
 
     <target> uses the same parameters as TexImage2D.
 
     <mailbox> identifies a GL_MAILBOX_SIZE_CHROMIUM byte sized name returned by
     glGenMailboxCHROMIUM.
@@ skipping 51 matching lines @@
 
     None.
 
 Revision History
 
     4/25/2011    Documented the extension
     5/23/2013    Major revision in Produce/Consume semantics, introducing
                  sharing.
     6/02/2014    Added glProduceTextureDirectCHROMIUM and
                  glCreateAndConsumeTextureCHROMIUM definitions.
+    8/03/2016    Allow unbinding mailbox using glProduceTextureDirectCHROMIUM.