Ozone caca: Fix segfault during GPU initialization

After https://codereview.chromium.org/2270463002,
gl::init::GetAllowedGLImplementations assumes GetSurfaceFactoryOzone to
be non-null on the GPU side. The remaining Ozone platform violating
this assumption is addressed in this CL.

BUG=640613
R=rjkroege@chromium.org

Review-Url: https://codereview.chromium.org/2346433002
Cr-Commit-Position: refs/heads/master@{#418781}

ui/ozone/platform/caca/ozone_platform_caca.cc

@@ -73,6 +73,12 @@ class OzonePlatformCaca : public OzonePlatform {
   }
 
   void InitializeGPU() override {
+    if (!window_manager_) {
+      // The return value of GetSurfaceFactoryOzone() must be non-null so a
+      // dummy instance of CacaWindowManager is needed to make the GPU
+      // initialization gracefully fail.
+      window_manager_.reset(new CacaWindowManager);
+    }
   }
 
  private: