win: Use target_out_dir instead of source_out_dir for pdbname

source_out_dir doesn't get set by GN per-cc as a variable when it's
filled out in a tool, whereas target_out_dir does for the target. As
this is only for a pdb name, (unlikely) collisions aren't a problem
anyway, so use that name instead. This is also what Chromium's toolchain
definition uses.

(Noticed in https://chromium-review.googlesource.com/996715 that Windows
bots are trying to write to E:\blah.pdb, and failing.)

Bug: crashpad:1
Change-Id: I548044333677bb592bf7a4520e787428e192dfdb
Reviewed-on: https://chromium-review.googlesource.com/996810
Reviewed-by: Mark Mentovai <mark@chromium.org>
diff --git a/build/BUILD.gn b/build/BUILD.gn
index e9ff6ee..f67f075 100644
--- a/build/BUILD.gn
+++ b/build/BUILD.gn
@@ -411,7 +411,7 @@
 
       tool("cc") {
         depfile = "{{output}}.d"
-        pdbname = "{{source_out_dir}}/{{label_name}}_c.pdb"
+        pdbname = "{{target_out_dir}}/{{label_name}}_c.pdb"
         command = "ninja -t msvc -e $env -- $cc /nologo /showIncludes {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} /c {{source}} /Fo{{output}} /Fd\"$pdbname\""
         depsformat = "msvc"
         description = "CC {{output}}"
@@ -422,7 +422,7 @@
 
       tool("cxx") {
         depfile = "{{output}}.d"
-        pdbname = "{{source_out_dir}}/{{label_name}}_cc.pdb"
+        pdbname = "{{target_out_dir}}/{{label_name}}_cc.pdb"
         command = "ninja -t msvc -e $env -- $cxx /nologo /showIncludes {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} /c {{source}} /Fo{{output}} /Fd\"$pdbname\""
         depsformat = "msvc"
         description = "CXX {{output}}"