Count the line pos, offset and size of compilation message in UTF-16

This patch counts the line position, offset and size of the compilation
message in UTF-16 and saves them to WGPUCompilationMessage to align the
latest WebGPU SPEC.

Bug: dawn:1357
Change-Id: If8f4026bd5b4a64a078e100762b6d1f61da50053
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/115640
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/tint/text/unicode.cc

@@ -427,6 +427,10 @@ std::pair<CodePoint, size_t> Decode(const uint8_t* ptr, size_t len) {
     return {c, n};
 }
 
+std::pair<CodePoint, size_t> Decode(std::string_view utf8_string) {
+    return Decode(reinterpret_cast<const uint8_t*>(utf8_string.data()), utf8_string.size());
+}
+
 bool IsASCII(std::string_view str) {
     for (auto c : str) {
         if (c & 0x80) {

src/tint/text/unicode.h

@@ -69,6 +69,12 @@ namespace utf8 {
 ///          If the next code point cannot be decoded then returns [0,0].
 std::pair<CodePoint, size_t> Decode(const uint8_t* ptr, size_t len);
 
+/// Decodes the first code point in the utf8 string.
+/// @param utf8_string the string view that contains the utf8 sequence
+/// @returns a pair of CodePoint and width in code units (bytes).
+///          If the next code point cannot be decoded then returns [0,0].
+std::pair<CodePoint, size_t> Decode(std::string_view utf8_string);
+
 /// @returns true if all the utf-8 code points in the string are ASCII
 /// (code-points 0x00..0x7f).
 bool IsASCII(std::string_view);