[canvaskit] Fix conversion of large images to base64

Bug: skia:
Change-Id: I8ab40a382557ae6bd13884ddce027e51ce1fb21f
Reviewed-on: https://skia-review.googlesource.com/c/173701
Reviewed-by: Kevin Lubick <kjlubick@google.com>

experimental/canvaskit/htmlcanvas/canvas2d.js

@@ -17,7 +17,21 @@
     if (isNode) {
       return Buffer.from(bytes).toString('base64');
     } else {
-      return btoa(String.fromCharCode.apply(null, bytes));
+      // From https://stackoverflow.com/a/25644409
+      // because the naive solution of
+      //     btoa(String.fromCharCode.apply(null, bytes));
+      // would occasionally throw "Maximum call stack size exceeded"
+      var CHUNK_SIZE = 0x8000; //arbitrary number
+      var index = 0;
+      var length = bytes.length;
+      var result = '';
+      var slice;
+      while (index < length) {
+        slice = bytes.slice(index, Math.min(index + CHUNK_SIZE, length));
+        result += String.fromCharCode.apply(null, slice);
+        index += CHUNK_SIZE;
+      }
+      return btoa(result);
     }
   }