Pass dsl::PositionInfo by value

We were previously using a mix of pass-by-value and pass-by-pointer (to
allow for explicitly null PositionInfo). Being able to pass a null
PositionInfo didn't really add much, since we can just use a nullary-
constructor PositionInfo instead, so these have all been migrated to
by-value.

Change-Id: Ia31e252cac94f64c4b38c29a54e6e7f752e70672
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/437276
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
diff --git a/tests/SkSLDSLTest.cpp b/tests/SkSLDSLTest.cpp
index c961115..94095f7 100644
--- a/tests/SkSLDSLTest.cpp
+++ b/tests/SkSLDSLTest.cpp
@@ -64,7 +64,7 @@
         SetErrorHandler(nullptr);
     }
 
-    void handleError(const char* msg, PositionInfo* pos) override {
+    void handleError(const char* msg, PositionInfo pos) override {
         REPORTER_ASSERT(fReporter, !strcmp(msg, fMsg),
                         "Error mismatch: expected:\n%sbut received:\n%s", fMsg, msg);
         fMsg = nullptr;