fix switch fallthrough warning

Bug: skia:
Change-Id: Ia2e25545319d54c3258b76c050b46a73c292c791
Reviewed-on: https://skia-review.googlesource.com/115085
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>

src/sksl/SkSLParser.cpp

@@ -1449,6 +1449,7 @@ std::unique_ptr<ASTExpression> Parser::assignmentExpression() {
                 result = std::unique_ptr<ASTExpression>(new ASTBinaryExpression(std::move(result),
                                                                                 std::move(t),
                                                                                 std::move(right)));
+                return result;
             }
             default:
                 return result;