Introduce String::fromUTF8Lenient() and use it for cache_name in CacheStorage API.

third_party/WebKit/Source/wtf/text/WTFStringTest.cpp

Index: third_party/WebKit/Source/wtf/text/WTFStringTest.cpp
diff --git a/third_party/WebKit/Source/wtf/text/WTFStringTest.cpp b/third_party/WebKit/Source/wtf/text/WTFStringTest.cpp
index 319320f0341fb6e9908cd03ea3596f5b4b007c87..51086350ef8e0d4f61143f8ed90037995f6b9df4 100644
--- a/third_party/WebKit/Source/wtf/text/WTFStringTest.cpp
+++ b/third_party/WebKit/Source/wtf/text/WTFStringTest.cpp
@@ -401,4 +401,17 @@ TEST(StringTest, Lower)
     EXPECT_STREQ("link", String::fromUTF8("LIN\xE2\x84\xAA").lower().utf8().data());
 }
 
+TEST(StringTest, fromUTF8Lenient)

[jsbell, 2016/03/08 20:28:15]

Nit: test name capitalized?

(I know this matches the method name, but that'll be changing soon and this is
really testing round-tripping, not just that method)

[horo, 2016/03/09 01:58:40]

Done.

+{
+    UChar buffer[] = {0x0061 /* 'a' */, 0x0062 /* 'b' */, 0x0063 /* 'c' */, 0xd800 /* surrogate value */};
+    String utf16String(buffer, sizeof(buffer) / sizeof(UChar));
+    ASSERT_FALSE(utf16String.isNull());
+
+    std::string utf8String(utf16String.utf8().data());
+    ASSERT_TRUE(String::fromUTF8(utf8String.data()).isNull());
+    String utf16NewString = String::fromUTF8Lenient(utf8String.data(), utf8String.length());

[jsbell, 2016/03/08 20:28:15]

In addition to testing that fromUTF8Lenient lets these through whereas fromUTF8
returns empty string, we might as well go ahead and test that all 64k code units
make it through cleanly.

I'd also like to see test cases for:

* NUL (U+0000)
* non-ASCII 
* unpaired high surrogate (0xDC00)
* word-swapped surrogates (0xDC00 0xD800)
* a correct surrogate pair (e.g. U+1F4A9 - 0xD83D 0xDCA9)
* non-characters (U+FFFF, etc)

(The multi-unit sequences above wouldn't be exercised by just doing all 64k code
units.)

[horo, 2016/03/09 01:58:40]

Done.

+    ASSERT_FALSE(utf16NewString.isNull());
+    ASSERT_EQ(utf16String, utf16NewString);
+}
+
 } // namespace WTF