<ostream> -> <iosfwd>

<ostream> is one of the more expensive headers to include
and that's amplified by SkRefCnt.h's popularity.

We've been including <ostream> for sk_sp's operator<<.  That's only
used by Chromium and while we could just sprinkle in a bunch of .get()
calls and remove operator<<, when I started going through and actually
doing that I got the feeling I was making things pointlessly harder to
read and write, and wanted to find a way to make it actually work.

My next instinct was to template it without mentioning ostreams,

    template <typename OS, typename T>
    auto operator<<(OS& os, const sk_sp<T>& sp) -> decltype(os << sp.get()) {
        return os << sp.get();
    }

but that makes this operator<< ambiguous with some other templated operator<<
in GTest.  They got in first, so they win...

So ultimately, switch <ostream> to <iosfwd>.   Anyone using our
operator<<() presumably has <ostream> included already, and the #include
cost for <iosfwd> is small enough that I don't think we'll mind keeping
this around indefinitely.

To repro, look at before/after of -ftime-trace:

    ~/chromium/src/third_party/llvm-build/Release+Asserts/bin/clang++ -I. -Os -c src/core/SkCanvas.cpp -ftime-trace

I have tested locally that Chromium builds with this change.

Change-Id: I9decc2e65b5cc8fd07d8106a5eff81901aedd7d5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/237190
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

include/core/SkRefCnt.h

@@ -13,8 +13,8 @@
 #include <atomic>
 #include <cstddef>
 #include <functional>
+#include <iosfwd>
 #include <memory>
-#include <ostream>
 #include <type_traits>
 #include <utility>
 

modules/skottie/src/SkottieTest.cpp

@@ -19,6 +19,7 @@
 #include "tools/ToolUtils.h"
 
 #include <cmath>
+#include <string>
 #include <tuple>
 #include <vector>
 

modules/skparagraph/include/ParagraphStyle.h

@@ -5,6 +5,7 @@
 #include "include/core/SkFontStyle.h"
 #include "modules/skparagraph/include/DartTypes.h"
 #include "modules/skparagraph/include/TextStyle.h"
+#include <string>  // std::u16string
 
 namespace skia {
 namespace textlayout {

src/core/SkVM.cpp

@@ -15,6 +15,10 @@
 #if defined(SKVM_JIT)
     #include <sys/mman.h>
 #endif
+#if defined(SKVM_PERF_DUMPS)
+    #include <stdio.h>
+    #include <time.h>
+#endif
 
 namespace skvm {