util/net: Place death tests into a *DeathTest test case.
util/net: Place death tests into a *DeathTest test case.

Not doing this causes gtest to issue these warnings:

[WARNING] ../../third_party/gtest/gtest/src/gtest-death-test.cc:825::
Death tests use fork(), which is unsafe particularly in a threaded
context. For this test, Google Test detected 4 threads.

The gtest documentation recommends giving the test case a name ending in
DeathTest. Test cases named according to this convention run before all
other tests. Other death tests in Crashpad also follow this convention.

https://code.google.com/p/googletest/wiki/AdvancedGuide#Death_Tests_And_Threads

TEST=util_test HTTPMultipartBuilderDeathTest.*
R=rsesek@chromium.org

Review URL: https://codereview.chromium.org/694963002

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