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/tests/PathOpsExtendedTest.cpp b/tests/PathOpsExtendedTest.cpp
index 27baa8c..c96cbcd 100644
--- a/tests/PathOpsExtendedTest.cpp
+++ b/tests/PathOpsExtendedTest.cpp
@@ -331,7 +331,7 @@
     }
 }
 
-static SkMutex compareDebugOut3;
+SK_DECLARE_STATIC_MUTEX(compareDebugOut3);
 
 static int comparePaths(skiatest::Reporter* reporter, const char* testName, const SkPath& one,
         const SkPath& scaledOne, const SkPath& two, const SkPath& scaledTwo, SkBitmap& bitmap,
@@ -419,7 +419,7 @@
     outFile.flush();
 }
 
-static SkMutex simplifyDebugOut;
+SK_DECLARE_STATIC_MUTEX(simplifyDebugOut);
 
 bool testSimplify(SkPath& path, bool useXor, SkPath& out, PathOpsThreadState& state,
                   const char* pathStr) {
@@ -578,7 +578,7 @@
     return true;
 }
 
-static SkMutex gMutex;
+SK_DECLARE_STATIC_MUTEX(gMutex);
 
 void initializeTests(skiatest::Reporter* reporter, const char* test) {
 #if 0  // doesn't work yet