Android: fix memory regression on svelte device

Virtualized GL contexts were being disabled for
some non-full screen video due to the attribute
|alpha| used for context creation requests made in
RenderWidget/RenderThreadImp was true.

This CL check the surface type (on/off) to disable
virtualized GL only for onscreen surfaces whose
format doesn't match the default.

BUG=594083, 591100

Review URL: https://codereview.chromium.org/1818303002

Cr-Commit-Position: refs/heads/master@{#383179}

content/common/gpu/gpu_command_buffer_stub.cc

@@ -243,7 +243,8 @@ GpuCommandBufferStub::GpuCommandBufferStub(
     surface_format_ = gfx::GLSurface::SURFACE_RGB565;
   gfx::GLSurface* defaultOffscreenSurface =
       channel_->gpu_channel_manager()->GetDefaultOffscreenSurface();
-  if (surface_format_ != defaultOffscreenSurface->GetFormat())
+  bool is_onscreen = (surface_handle_ != gpu::kNullSurfaceHandle);
+  if (surface_format_ != defaultOffscreenSurface->GetFormat() && is_onscreen)
     use_virtualized_gl_context_ = false;
 #endif