ProfileEntry has |string|, which can be static or dynamic, and |dynamicString|.
If |string| is dynamic, the FRAME_LABEL_COPY flag must be set, and it will be
copied into profiler output.
But there is only one place that uses dynamic |string| values, in SpiderMonkey.
And that place doesn't use |dynamicString|. So this patch changes that place to
use an empty |string| and put the old dynamic |string| value in
|dynamicString|. This in turn removes the need for FRAME_LABEL_COPY.
One minor wrinkle is that when |dynamicString| is used the old code put a space
between |string| and |dynamicString|. The new code omits the space if |string|
is empty.
The patch also renames ProfileEntry::string as ProfileEntry::label_, which
better matches how it's used, and ProfileEntry::dynamicString as
ProfileEntry::dynamicString_ so the getter can be renamed dynamicString().
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