Adds a new class HttpCache::Writers for multiple cache transactions reading from the network.

net/BUILD.gn

Index: net/BUILD.gn
diff --git a/net/BUILD.gn b/net/BUILD.gn
index 6b6dbf8297db54f0a4a407a0e15078921cfc818e..916a0aeb2439038c7ec6332f2dcc3cce42899c48 100644
--- a/net/BUILD.gn
+++ b/net/BUILD.gn
@@ -837,6 +837,8 @@ component("net") {
       "http/http_cache_lookup_manager.h",
       "http/http_cache_transaction.cc",
       "http/http_cache_transaction.h",
+      "http/http_cache_writers.cc",
+      "http/http_cache_writers.h",
       "http/http_chunked_decoder.cc",
       "http/http_chunked_decoder.h",
       "http/http_content_disposition.cc",
@@ -4457,6 +4459,7 @@ test("net_unittests") {
     "http/http_byte_range_unittest.cc",
     "http/http_cache_lookup_manager_unittest.cc",
     "http/http_cache_unittest.cc",
+    "http/http_cache_writers_unittest.cc",
     "http/http_chunked_decoder_unittest.cc",
     "http/http_content_disposition_unittest.cc",
     "http/http_log_util_unittest.cc",