gpu: Mark textures immutable for TexStorage2DImageCHROMIUM

TexStorage2DImageCHROMIUM should create an immutable texture like the
TexStorage function it's modelled after.

Change-Id: I8cde40d64df307d863974401c8eeaff69d5b56ae
Reviewed-on: https://chromium-review.googlesource.com/847966Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Kristian H. Kristensen <hoegsberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526842}

gpu/command_buffer/service/gles2_cmd_decoder.cc

@@ -18112,6 +18112,8 @@ void GLES2DecoderImpl::DoTexStorage2DImageCHROMIUM(GLenum target,
 
   if (texture->IsAttachedToFramebuffer())
     framebuffer_state_.clear_state_dirty = true;
+
+  texture->SetImmutable(true);
 }
 
 void GLES2DecoderImpl::DoProduceTextureDirectCHROMIUM(