Add missing dumpfile.cc to leveldatabase target in GN build.

This was found by looking at the output of gyp_flag_compare step in
http://build.chromium.org/p/chromium.linux/builders/Linux%20GN%20%28dbg%29/builds/24157/steps/gyp_flag_compare/logs/stdio

Tested on Linux with the following command lines:

$ gn gen out-gn
$ ninja -C out-gn leveldatabase
$ touch third_party/leveldatabase/src/db/dumpfile.cc
$ ninja -C out-gn leveldatabase

BUG=461019
TEST=see above
R=cmumford@chromium.org
TBR=brettw@chromium.org

CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:android_chromium_gn_compile_dbg,android_chromium_gn_compile_rel;tryserver.chromium.win:win8_chromium_gn_rel,win8_chromium_gn_dbg;tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg

Review URL: https://codereview.chromium.org/1044613002

Cr-Original-Commit-Position: refs/heads/master@{#322962}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8a33c992e0a969f8ec7584da579980dba660e376

secondary/third_party/leveldatabase/BUILD.gn

@@ -34,6 +34,7 @@ static_library("leveldatabase") {
     "src/db/db_iter.h",
     "src/db/dbformat.cc",
     "src/db/dbformat.h",
+    "src/db/dumpfile.cc",
     "src/db/filename.cc",
     "src/db/filename.h",
     "src/db/log_format.h",
@@ -59,6 +60,7 @@ static_library("leveldatabase") {
     "src/include/leveldb/cache.h",
     "src/include/leveldb/comparator.h",
     "src/include/leveldb/db.h",
+    "src/include/leveldb/dumpfile.h",
     "src/include/leveldb/env.h",
     "src/include/leveldb/filter_policy.h",
     "src/include/leveldb/iterator.h",