Fixed DSL build when threadlocals are not available

This version of the method was only used when threadlocals are
unavailable, so our normal buildbots didn't catch the name mismatch.

TBR=brianosman@google.com,johnstiles@google.com

Change-Id: I58ed254c8bca62b1aa56ac26ca767d91eb67458c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/439284
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>

src/sksl/dsl/priv/DSLWriter.cpp

@@ -322,7 +322,7 @@ static pthread_key_t get_pthread_key() {
     return sKey;
 }
 
-bool DSLWriter::Active() {
+bool DSLWriter::IsActive() {
     return pthread_getspecific(get_pthread_key()) != nullptr;
 }