Check for sharedgl on loseSharedGraphicsContext3D

This was causing a null read on testing.

Bug: 1068640
Change-Id: I88f1e60284484b9f88adf87302deda4e86b594cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2152886
Auto-Submit: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759834}

third_party/blink/renderer/core/testing/internals.cc

@@ -3010,6 +3010,8 @@ bool Internals::loseSharedGraphicsContext3D() {
   if (!shared_provider)
     return false;
   gpu::gles2::GLES2Interface* shared_gl = shared_provider->ContextGL();
+  if (!shared_gl)
+    return false;
   shared_gl->LoseContextCHROMIUM(GL_GUILTY_CONTEXT_RESET_EXT,
                                  GL_INNOCENT_CONTEXT_RESET_EXT);
   // To prevent tests that call loseSharedGraphicsContext3D from being