Bug 1365854 - Remove FRAME_LABEL_COPY. r=mstange,shu.

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().

xpcom/threads/ThreadStackHelper.cpp

@@ -497,11 +497,6 @@ ThreadStackHelper::FillStackBuffer()
   // Deduplicate identical, consecutive frames
   const char* prevLabel = nullptr;
   for (; reservedSize-- && entry != end; entry++) {
-    /* We only accept non-copy labels, including js::RunScript,
-       because we only want static labels in the hang stack. */
-    if (entry->isCopyLabel()) {
-      continue;
-    }
     if (entry->isJs()) {
       prevLabel = AppendJSEntry(entry, availableBufferSize, prevLabel);
       continue;