add replace_char() method to SkString API (skbug.com/2651)

tests/StringTest.cpp

Index: tests/StringTest.cpp
diff --git a/tests/StringTest.cpp b/tests/StringTest.cpp
index 13ee5ec2153ff6da775bef64887fc16f759da2f4..5bf49b17b45eeaa0fbca2c77f46e04377db26b94 100644
--- a/tests/StringTest.cpp
+++ b/tests/StringTest.cpp
@@ -186,6 +186,23 @@ DEF_TEST(String, reporter) {
 
 }
 
+DEF_TEST(String_ReplaceChar, r) {
+    a.set("hello world");
+    a.replaceChar('h', 'h');
+    REPORTER_ASSERT(reporter, a.equals("hello world"));
+    a.replaceChar('x', 'x');
+    REPORTER_ASSERT(reporter, a.equals("hello world"));
+    a.replaceChar('o', 'e');
+    REPORTER_ASSERT(reporter, a.equals("helle werld"));
+    a.replaceChar('x', 'e');
+    REPORTER_ASSERT(reporter, a.equals("helle werld"));
+    a.replaceChar('x', 'z');
+    REPORTER_ASSERT(reporter, a.equals("helle werld"));
+    a.set("");
+    a.replaceChar('a', 'b');
+    REPORTER_ASSERT(reporter, a.equals(""));
+}
+
 DEF_TEST(String_SkStrSplit, r) {
     SkTArray<SkString> results;