[Android] Add support for standalone PDFium gn build on Android.

build_overrides/build.gni

Index: build_overrides/build.gni
diff --git a/build_overrides/build.gni b/build_overrides/build.gni
index 9d68f109332bfc2c8c81676a9a076891cc4f773c..1c0b53fe19cfda2bba0a90b21a00a6489c04a87e 100644
--- a/build_overrides/build.gni
+++ b/build_overrides/build.gni
@@ -3,6 +3,20 @@
 # found in the LICENSE file.
 
 # See https://bugs.chromium.org/p/webrtc/issues/detail?id=5453.
-# These overrides are needed and should generally track Chromium settings.
-mac_deployment_target_build_override = "10.7"
+# Some WebRTC targets require the 10.7 deployment version of the Mac SDK
+# and a 10.11 min SDK, but those targets are only used in non-Chromium
+# builds. We can remove this when Chromium drops 10.6 support and also
+# requires 10.7.
 mac_sdk_min_build_override = "10.10"
+mac_deployment_target_build_override = "10.7"
+
+# Variable that can be used to support multiple build scenarios, like having
+# Chromium specific targets in a client project's GN file etc.
+build_with_chromium = true
+
+# Support different NDK locations in non-Chromium builds.
+default_android_ndk_root = "//third_party/android_ndk"
+default_android_ndk_version = "r11c"
+
+# Some non-Chromium builds don't support building java targets.
+enable_java_templates = false