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A reference to capturing_net_log_ was passed into the base class constructor
and used before it was fully initialized. For the derived class the initialization
of the members happens stronly after the base constructors completed.

This fixes a bug found by UBSan Vptr.

BUG=617206

Review-Url: https://codereview.chromium.org/2042493002
Cr-Commit-Position: refs/heads/master@{#397846}