Make WebRTC work with Chromium Git checkouts

WebRTC standalone shares a lot of dependencies and build
tools with Chromium. To make the build work, many of the
paths of a Chromium checkout is now emulated by creating
symlinks to files and directories.

All DEPS entries that previously used Var("chromium_trunk")
to reference a Chromium checkout or From("chromium_deps"..)
to reference the Chromium DEPS file are now removed and
replaced by symlink entries in setup_links.py.

The script also handles cleanup of the legacy
Subversion-based dependencies that's needed for the
transition.

Windows: One Windows-specific important change is that
gclient sync|runhooks must now be run from a shell
with Administrator privileges in order to be able to create
symlinks. This also means that Windows XP is no longer
supported.

To transition a previously created checkout:
Run "python setup_links.py --force" to cleanup the old
SVN-based dependencies that have been synced by gclient sync.
For Buildbots, the --force flag is automatically enabled for
their syncs.

BUG=2863, chromium:339647
TEST=Manual testing on Linux, Mac and Windows.
R=andrew@webrtc.org, iannucci@chromium.org, phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18379005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6938 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/.gitignore b/.gitignore
index 96556d7..64ccdd5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -37,10 +37,13 @@
 /Makefile
 /build
 /buildtools
-/chromium_deps
-/chromium_gn
-/google_apis/build
+/chromium/.gclient_entries
+/chromium/.last_sync_chromium
+/chromium/src
+/google_apis
 /gyp-mac-tool
+/links
+/links.db
 /net
 /out
 /resources/*.*
@@ -56,12 +59,12 @@
 /third_party/asan
 /third_party/binutils
 /third_party/boringssl
-/third_party/build_gn
 /third_party/BUILD.gn
 /third_party/clang_format
 /third_party/colorama
 /third_party/cygwin
 /third_party/directxsdk
+/third_party/drmemory
 /third_party/expat
 /third_party/gaeunit
 /third_party/gflags/src
@@ -85,7 +88,7 @@
 /third_party/opus
 /third_party/protobuf
 /third_party/sqlite
-/third_party/syzygy/binaries
+/third_party/syzygy
 /third_party/usrsctp
 /third_party/valgrind
 /third_party/winsdk_samples/src
@@ -93,7 +96,7 @@
 /tools/android
 /tools/android-dummy-test
 /tools/clang
-/tools/find_depot_tools
+/tools/find_depot_tools.py
 /tools/generate_library_loader
 /tools/gn
 /tools/grit