fix race: this kCacheSize is probably not meant to be static

TSAN sees us racing to intialize the static const kCacheSize.

This static const value depends on the runtime value image->getSize(), so there
is a race to set it.  I think this is unintentionally copy-paste from the other
tests that use a constant kCacheSize.  It's weird to intialize a constant based
on the first call to test_internal_purge(), though luckily we're always calling
it with same sized images today.

See these TSAN failures:
    https://uberchromegw.corp.google.com/i/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-TSAN/builds/4937
    https://uberchromegw.corp.google.com/i/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-TSAN/builds/4940/steps/dm/logs/stdio

TBR=robertphillips@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1720743002

Review URL: https://codereview.chromium.org/1720743002
diff --git a/tests/ImageFilterCacheTest.cpp b/tests/ImageFilterCacheTest.cpp
index 175d17c..74c56df 100644
--- a/tests/ImageFilterCacheTest.cpp
+++ b/tests/ImageFilterCacheTest.cpp
@@ -78,7 +78,7 @@
 // Test purging when the max cache size is exceeded
 static void test_internal_purge(skiatest::Reporter* reporter, SkSpecialImage* image) {
     SkASSERT(image->getSize());
-    static const size_t kCacheSize = image->getSize() + 10;
+    const size_t kCacheSize = image->getSize() + 10;
     SkAutoTUnref<SkImageFilter::Cache> cache(SkImageFilter::Cache::Create(kCacheSize));
 
     SkIRect clip = SkIRect::MakeWH(100, 100);
@@ -219,14 +219,14 @@
     const SkIRect& full = SkIRect::MakeWH(kFullSize, kFullSize);
 
     SkAutoTUnref<SkSpecialImage> fullImg(SkSpecialImage::NewFromGpu(
-                                                                nullptr, full, 
+                                                                nullptr, full,
                                                                 kNeedNewImageUniqueID_SpecialImage,
                                                                 srcTexture));
 
     const SkIRect& subset = SkIRect::MakeXYWH(kPad, kPad, kSmallerSize, kSmallerSize);
 
     SkAutoTUnref<SkSpecialImage> subsetImg(SkSpecialImage::NewFromGpu(
-                                                                nullptr, subset, 
+                                                                nullptr, subset,
                                                                 kNeedNewImageUniqueID_SpecialImage,
                                                                 srcTexture));