Fix dangling reference in NormalSpdyTransactionHelper.

NormalSpdyTransactionHelper constructor is often (approximately 70 times) called
with a temporary BoundNetLog() argument.  Its log_ member currently takes
reference of that, which becomes dangling right after the constructor returns.

This is not an issue if there are no NetLog events, nor in the NetLog test which
creates a named CapturingBoundNetLog object that it keeps in scope.  The
dangling reference only became problematic in the HTTP11RequiredProxyRetry test,
and curiously enough, only for 64 bit Andoid builds.

This CL changes the NormalSpdyTransactionHelper.log_ member type from reference
to value to fix this issue.

BUG=447837

Review URL: https://codereview.chromium.org/852053002

Cr-Commit-Position: refs/heads/master@{#311538}

net/spdy/spdy_network_transaction_unittest.cc

@@ -417,7 +417,7 @@ class SpdyNetworkTransactionTest
     DataVector data_vector_;
     AlternateVector alternate_vector_;
     AlternateDeterministicVector alternate_deterministic_vector_;
-    const BoundNetLog& log_;
+    const BoundNetLog log_;
     SpdyNetworkTransactionTestParams test_params_;
     int port_;
     bool deterministic_;