[cronet] make startNetLogToFile write to correct file

components/cronet/ios/Cronet.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "components/cronet/ios/Cronet.h"
 
 #include <memory>
 
 #include "base/lazy_instance.h"
 #include "base/logging.h"
@@ skipping 212 matching lines @@
     [NSURLCache setSharedURLCache:gPreservedSharedURLCache];
     gPreservedSharedURLCache = nil;
   }
   [NSURLProtocol unregisterClass:[CRNPauseableHTTPProtocolHandler class]];
 }
 
 + (void)installIntoSessionConfiguration:(NSURLSessionConfiguration*)config {
   config.protocolClasses = @[ [CRNPauseableHTTPProtocolHandler class] ];
 }
 
-+ (void)startNetLogToFile:(NSString*)fileName logBytes:(BOOL)logBytes {
-  if (gChromeNet.Get().get() && [fileName length]) {
-    gChromeNet.Get()->StartNetLog([fileName UTF8String], logBytes);
++ (NSString*)getNetLogPathForFile:(NSString*)fileName {
+  return [[[[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory
+                                                   inDomains:NSUserDomainMask]
+      lastObject] URLByAppendingPathComponent:fileName] path];
+}
+
++ (BOOL)startNetLogToFile:(NSString*)fileName logBytes:(BOOL)logBytes {
+  if (gChromeNet.Get().get() && [fileName length] &&
+      ![fileName isAbsolutePath]) {
+    return gChromeNet.Get()->StartNetLog(
+        base::SysNSStringToUTF8([self getNetLogPathForFile:fileName]),
+        logBytes);
   }
+
+  return NO;
 }
 
 + (void)stopNetLog {
   if (gChromeNet.Get().get()) {
     gChromeNet.Get()->StopNetLog();
   }
 }
 
 + (NSString*)getUserAgent {
   if (!gChromeNet.Get().get()) {
@@ skipping 32 matching lines @@
       base::SysNSStringToUTF8(hostResolverRulesForTesting));
 }
 
 // This is a non-public dummy method that prevents the linker from stripping out
 // the otherwise non-referenced methods from 'bidirectional_stream.cc'.
 + (void)preventStrippingCronetBidirectionalStream {
   bidirectional_stream_create(NULL, 0, 0);
 }
 
 @end