Revert of SK_DECLARE_STATIC_MUTEX -> static SkMutex (patchset #1 id:1 of https://codereview.chromium.org/1948193002/ )

Reason for revert:
broken the Mac and Linux builders, e.g.:

https://build.chromium.org/p/chromium/builders/Mac/builds/15151
https://build.chromium.org/p/chromium/builders/Linux%20x64/builds/19052

Original issue's description:
> SK_DECLARE_STATIC_MUTEX -> static SkMutex
>
> There's no need to use a macro to declare static SkMutexes any more
> (and there's likewise no need to restrict them to global scope).
>
> BUG=skia:
> GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1948193002
>
> Committed: https://skia.googlesource.com/skia/+/5e56cfd3fa1041dbb83899844fb92fa9a2ef1009

TBR=mtklein@google.com,mtklein@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review-Url: https://codereview.chromium.org/1945353003
diff --git a/dm/DMJsonWriter.cpp b/dm/DMJsonWriter.cpp
index 652c70b..7bd8acd 100644
--- a/dm/DMJsonWriter.cpp
+++ b/dm/DMJsonWriter.cpp
@@ -19,7 +19,7 @@
 namespace DM {
 
 SkTArray<JsonWriter::BitmapResult> gBitmapResults;
-static SkMutex gBitmapResultLock;
+SK_DECLARE_STATIC_MUTEX(gBitmapResultLock);
 
 void JsonWriter::AddBitmapResult(const BitmapResult& result) {
     SkAutoMutexAcquire lock(&gBitmapResultLock);
@@ -27,7 +27,7 @@
 }
 
 SkTArray<skiatest::Failure> gFailures;
-static SkMutex gFailureLock;
+SK_DECLARE_STATIC_MUTEX(gFailureLock);
 
 void JsonWriter::AddTestFailure(const skiatest::Failure& failure) {
     SkAutoMutexAcquire lock(gFailureLock);