media::learning: Make LabelledExample's move assignment operator noexcept

The GCC build is currently broken with an error like this:

    ../../media/learning/common/labelled_example.cc:20:1: error: function ‘media::learning::LabelledExample::LabelledExample(media::learning::LabelledExample&&)’ defaulted on its redeclaration with an exception-specification that differs from the implicit exception-specification ‘’
     LabelledExample::LabelledExample(LabelledExample&& rhs) noexcept = default;
     ^~~~~~~~~~~~~~~

With GCC, having that noexcept marker requires all members to be marked with
noexcept themselves, and TargetValue was missing some assignment operators
and noexcept markers.

clang is fine because we pass -fno-exceptions and it disables the same error
there, while GCC continues to raise it (bug 843143 and its corresponding CL
have a longer discussion on this issue).

Bug: 819294
Change-Id: Ide30932fc466ccb52d6883a82777e703dae48798
Reviewed-on: https://chromium-review.googlesource.com/c/1458210
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#630355}
4 files changed