base::Optional -> absl::optional

[jellefoks]

Replace base::Optional use with asbl::optional.

b/315503544

[jellefoks]

To be addressed still here:

[38.216 sec | 9822/15605 5781 remaining | 43.8/sec | j3] LINK ./base_unittests
FAILED: base_unittests 
"python3" "../../build/toolchain/gcc_link_wrapper.py" --output="./base_unittests" -- /github/home/starboard-toolchains/x86_64-linux-gnu-clang-chromium-16-init-17653-g39da55e8-2/bin/clang++ -static-libstdc++ -lpthread -latomic -o "./base_unittests" -Wl,--start-group @"./base_unittests.rsp"  -Wl,--end-group 
/usr/bin/ld: obj/third_party/abseil-cpp/absl/log/internal/conditions/conditions.o: in function `absl::log_internal::LogEveryNSecState::ShouldLog(double)':
conditions.cc:(.text+0xa1): undefined reference to `absl::base_internal::UnscaledCycleClock::Frequency()'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[andrewsavage1]

To be addressed still here:

[38.216 sec | 9822/15605 5781 remaining | 43.8/sec | j3] LINK ./base_unittests
FAILED: base_unittests 
"python3" "../../build/toolchain/gcc_link_wrapper.py" --output="./base_unittests" -- /github/home/starboard-toolchains/x86_64-linux-gnu-clang-chromium-16-init-17653-g39da55e8-2/bin/clang++ -static-libstdc++ -lpthread -latomic -o "./base_unittests" -Wl,--start-group @"./base_unittests.rsp"  -Wl,--end-group 
/usr/bin/ld: obj/third_party/abseil-cpp/absl/log/internal/conditions/conditions.o: in function `absl::log_internal::LogEveryNSecState::ShouldLog(double)':
conditions.cc:(.text+0xa1): undefined reference to `absl::base_internal::UnscaledCycleClock::Frequency()'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

In the feature branch I fixed this with 676fd80 (#2549), though it's untested on windows.