Replace a lot of 'static const' with 'constexpr' or 'const'.

'static const' means, there must be at most one of these, and initialize it at
compile time if possible or runtime if necessary.  This leads to unexpected
code execution, and TSAN* will complain about races on the guard variables.

Generally 'constexpr' or 'const' are better choices.  Neither can cause races:
they're either intialized at compile time (constexpr) or intialized each time
independently (const).

This CL prefers constexpr where possible, and uses const where not.  It even
prefers constexpr over const where they don't make a difference... I want to have
lots of examples of constexpr for people to see and mimic.

The scoped-to-class static has nothing to do with any of this, and is not changed.

* Not yet on the bots, which use an older TSAN.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2300623005

Review-Url: https://codereview.chromium.org/2300623005
diff --git a/gm/tallstretchedbitmaps.cpp b/gm/tallstretchedbitmaps.cpp
index f22f2d2..b70ae3d 100644
--- a/gm/tallstretchedbitmaps.cpp
+++ b/gm/tallstretchedbitmaps.cpp
@@ -12,12 +12,12 @@
 #include "SkRandom.h"
 
 int make_bm(SkBitmap* bm, int height) {
-    static const int kRadius = 22;
-    static const int kMargin = 8;
-    static const SkScalar kStartAngle = 0;
-    static const SkScalar kDAngle = 25;
-    static const SkScalar kSweep = 320;
-    static const SkScalar kThickness = 8;
+    constexpr int kRadius = 22;
+    constexpr int kMargin = 8;
+    constexpr SkScalar kStartAngle = 0;
+    constexpr SkScalar kDAngle = 25;
+    constexpr SkScalar kSweep = 320;
+    constexpr SkScalar kThickness = 8;
 
     int count = (height / (2 * kRadius + kMargin));
     height = count * (2 * kRadius + kMargin);