[Promises] Drop constexpr from unique_any/any_internal's operator=

This fixes the GCC build:

    ../../base/containers/any_internal.h: In member function ‘constexpr void base::internal::AnyInternal::operator=(base::internal::AnyInternal&&)’:
    ../../base/containers/any_internal.h:42:10: error: call to non-‘constexpr’ function ‘void base::internal::AnyInternal::reset()’
         reset();
         ~~~~~^~

reset() itself is not constexpr, and it is not clear if it should be given
it calls |delete_fn_ptr|.

Bug: 819294, 906125
Change-Id: I7760c62213db0e59a82e5210988ee469b5006090
Reviewed-on: https://chromium-review.googlesource.com/c/1458096
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630029}
2 files changed