Building without incremental_chrome_dll=1 works:
1>------ Build started: Project: webkit, Configuration: Debug Win32 ------
1>Linking...
1>Microsoft (R) Incremental Linker Version 9.00.30729.01
1>Copyright (C) Microsoft Corporation. All rights reserved.
1>d:\chromium\src\build\Debug\obj\webkit\RSP00048E71202848.rsp : fatal error LNK1170: line in command file contains 131071 or more characters
1>Build log was saved at "file://d:\chromium\src\build\Debug\obj\webkit\BuildLog.htm"
1>webkit - 1 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 87 up-to-date, 0 skipped ==========