trace_arguments: Mark StringStorage's move operations noexcept

This fixes the GCC build after eebbd7f6 ("Introduce
base::trace_event::TraceArguments helper class"):

    ../../base/trace_event/trace_event_impl.cc:61:1: error: function ‘base::trace_event::TraceEvent::TraceEvent(base::trace_event::TraceEvent&&)’ defaulted on its redeclaration with an exception-specification that differs from the implicit exception-specification ‘’
     TraceEvent::TraceEvent(TraceEvent&& other) noexcept = default;
     ^~~~~~~~~~
    ../../base/trace_event/trace_event_impl.cc:62:13: error: function ‘base::trace_event::TraceEvent& base::trace_event::TraceEvent::operator=(base::trace_event::TraceEvent&&)’ defaulted on its redeclaration with an exception-specification that differs from the implicit exception-specification ‘’
     TraceEvent& TraceEvent::operator=(TraceEvent&& other) noexcept = default;
                 ^~~~~~~~~~

With GCC, having that noexcept marker requires all members to be marked
with noexcept themselves, and StringStorage was missing it.

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: Ie2eba30dcb6d4a5f9cc1b4b7c961af96d9f55f10
Reviewed-on: https://chromium-review.googlesource.com/c/1374230Reviewed-by: David Turner <digit@chromium.org>
Reviewed-by: oysteine <oysteine@chromium.org>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#616082}

base/trace_event/trace_arguments.h

@@ -462,11 +462,11 @@ class BASE_EXPORT StringStorage {
       ::free(data_);
   }
 
-  StringStorage(StringStorage&& other) : data_(other.data_) {
+  StringStorage(StringStorage&& other) noexcept : data_(other.data_) {
     other.data_ = nullptr;
   }
 
-  StringStorage& operator=(StringStorage&& other) {
+  StringStorage& operator=(StringStorage&& other) noexcept {
     if (this != &other) {
       if (data_)
         ::free(data_);