Replace str.as_string() with Http2String(str).

Also update some comments to refer to Http2String.

This CL lands server changes 189187480 by ahedberg and 190218258 by
diannahu.

BUG=488484

Change-Id: Iffc6a90651103954e2528f16a3d3f7e288d4f333
Reviewed-on: https://chromium-review.googlesource.com/978392Reviewed-by: Bence Béky <bnc@chromium.org>
Commit-Queue: Dianna Hu <diannahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545992}

net/http2/hpack/decoder/hpack_decoder_string_buffer.cc

@@ -8,6 +8,7 @@
 
 #include "base/logging.h"
 #include "base/trace_event/memory_usage_estimator.h"
+#include "net/http2/platform/api/http2_string.h"
 #include "net/http2/tools/http2_bug_tracker.h"
 
 namespace net {
@@ -166,7 +167,7 @@ void HpackDecoderStringBuffer::BufferStringIfUnbuffered() {
   DVLOG(3) << "HpackDecoderStringBuffer::BufferStringIfUnbuffered state="
            << state_ << ", backing=" << backing_;
   if (state_ != State::RESET && backing_ == Backing::UNBUFFERED) {
-    DVLOG(2) << "HpackDecoderStringBuffer buffering string of length "
+    DVLOG(2) << "HpackDecoderStringBuffer buffering Http2String of length "
              << value_.size();
     buffer_.assign(value_.data(), value_.size());
     if (state_ == State::COMPLETE) {
@@ -201,7 +202,7 @@ Http2String HpackDecoderStringBuffer::ReleaseString() {
     if (backing_ == Backing::BUFFERED) {
       return std::move(buffer_);
     } else {
-      return value_.as_string();
+      return Http2String(value_);
     }
   }
   return "";

net/http2/hpack/decoder/hpack_decoder_test.cc

@@ -121,7 +121,7 @@ class HpackDecoderTest : public ::testing::TestWithParam<bool>,
   // error_message may be used in a GOAWAY frame as the Opaque Data.
   void OnHeaderErrorDetected(Http2StringPiece error_message) override {
     ASSERT_TRUE(saw_start_);
-    error_messages_.push_back(error_message.as_string());
+    error_messages_.push_back(Http2String(error_message));
     // No further callbacks should be made at this point, so replace 'this' as
     // the listener with mock_listener_, which is a strict mock, so will
     // generate an error for any calls.

net/http2/http2_constants.h

@@ -53,10 +53,10 @@ inline bool IsSupportedHttp2FrameType(Http2FrameType v) {
   return IsSupportedHttp2FrameType(static_cast<uint32_t>(v));
 }
 
-// The return type is 'string' so that they can generate a unique string for
-// each unsupported value. Since these are just used for debugging/error
-// messages, that isn't a cost to we need to worry about.
-// The same applies to the functions later in this file.
+// The return type is 'Http2String' so that they can generate a unique string
+// for each unsupported value. Since these are just used for debugging/error
+// messages, that isn't a cost to we need to worry about. The same applies to
+// the functions later in this file.
 HTTP2_EXPORT_PRIVATE Http2String Http2FrameTypeToString(Http2FrameType v);
 HTTP2_EXPORT_PRIVATE Http2String Http2FrameTypeToString(uint8_t v);
 HTTP2_EXPORT_PRIVATE inline std::ostream& operator<<(std::ostream& out,