Fix a compilation warning.

Fixes this warning:

libweave/src/device_registration_info_unittest.cc:342:32: error: lambda capture 'this' is not used [-Werror,-Wunused-lambda-capture]
libweave-0.0.1-r249:   auto callback = [&succeeded, this](const base::DictionaryValue& info,

BUG=chromium:714984
TEST=Unit test compilation failure on amd64-generic-paladin goes away.

Change-Id: I08f1c7eb77796bef7e531ffa81be00967f2cfaf0
Reviewed-on: https://weave-review.googlesource.com/9950
Reviewed-by: Manoj Gupta <manojgupta@google.com>

src/device_registration_info_unittest.cc

diff --git a/src/device_registration_info_unittest.cc b/src/device_registration_info_unittest.cc
index 6ccf00e..1be7984 100644
--- a/src/device_registration_info_unittest.cc
+++ b/src/device_registration_info_unittest.cc
@@ -339,8 +339,8 @@
           })));
 
   bool succeeded = false;
-  auto callback = [&succeeded, this](const base::DictionaryValue& info,
-                                     ErrorPtr error) {
+  auto callback = [&succeeded](const base::DictionaryValue& info,
+                               ErrorPtr error) {
     EXPECT_FALSE(error);
     std::string id;
     EXPECT_TRUE(info.GetString("id", &id));