Fix crash in ContextProviderMojo due to uninitialized member variable.

mojo/cc/context_provider_mojo.cc

Index: mojo/cc/context_provider_mojo.cc
diff --git a/mojo/cc/context_provider_mojo.cc b/mojo/cc/context_provider_mojo.cc
index 0d27f7153c81aaa07f0537e008dd1371abd6da80..5b0a640287fd8176bf0f9cb776583b395876fcad 100644
--- a/mojo/cc/context_provider_mojo.cc
+++ b/mojo/cc/context_provider_mojo.cc
@@ -12,7 +12,8 @@ namespace mojo {
 
 ContextProviderMojo::ContextProviderMojo(
     ScopedMessagePipeHandle command_buffer_handle)
-    : command_buffer_handle_(command_buffer_handle.Pass()) {
+    : command_buffer_handle_(command_buffer_handle.Pass()),
+      context_(nullptr) {
   // Enabled the CHROMIUM_image extension to use GpuMemoryBuffers. The
   // implementation of which is used in CommandBufferDriver.
   capabilities_.gpu.image = true;