This removes all chromium dependencies from media/base/ and completes
the build system in CMake.
The ClosureThread class and its classes were removed, as they were
specific to chromium base. ClosureThread has been replaced by
std::thread.
The byte-swapping utilities in network_util.cc have been removed and
replaced with absl.
generate_unique_temp_path() was split out of file_unittest.cc into
file_test_util.cc, where other test suites could make use of it.
WARN_UNUSED_RESULT was replaced with the C++ standard attribute
[[nodiscard]].
The base::Clock interface was replaced with a typedef for a function
pointer that returns the current time.
This re-enables the tests in http_key_fetcher_unittest.cc by using
httpbin.org.
Issue #1047 (CMake porting)
Issue #346 (absl porting)
Joey Parrish