Add the ability to change directories before execing.

base/process/launch_posix.cc

Index: base/process/launch_posix.cc
diff --git a/base/process/launch_posix.cc b/base/process/launch_posix.cc
index ce93d5056fd838bf332da563186c5bc704130e22..921e263ae834bd4a0010f26623036ee81665e900 100644
--- a/base/process/launch_posix.cc
+++ b/base/process/launch_posix.cc
@@ -516,6 +516,10 @@ Process LaunchProcess(const std::vector<std::string>& argv,
 #endif
 
 #if defined(OS_POSIX)
+    if (!options.current_directory.empty()) {
+      RAW_CHECK(chdir(options.current_directory.value().c_str()) == 0);

[jln (very slow on Chromium), 2015/02/03 02:07:53]

Sorry I didn't comment on that before: you need to do the c_str() conversion
prefork to be async safe.

[rickyz (no longer on Chrome), 2015/02/03 02:17:56]

Sure, I moved another c_str caller up as well.

+    }
+
     if (options.pre_exec_delegate != nullptr) {
       options.pre_exec_delegate->RunAsyncSafe();
     }