Move Abseil to flutter/third_party/abseil-cpp (#831)

In combination with

https://flutter-review.googlesource.com/c/third_party/abseil-cpp/+/55848,
this updates Flutter's references to Abseil from
//third_party/abseil-cpp to //flutter/third_party/abseil-cpp.

Will be followed by a DEPS roll in flutter/engine and patches to our own
BUILD.gn files and #includes.

Issue: https://github.com/flutter/flutter/issues/144201
Part of: https://github.com/flutter/flutter/issues/67373

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advice.
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- [X] I listed at least one issue that this PR fixes in the description
above.
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Quebec. Void where prohibited or restricted by law.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
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diff --git a/build_overrides/angle.gni b/build_overrides/angle.gni
index a4eacc8..e1ad74b 100644
--- a/build_overrides/angle.gni
+++ b/build_overrides/angle.gni
@@ -18,7 +18,7 @@
 }
 
 # Overrides for ANGLE's dependencies.
-angle_abseil_cpp_dir = "//third_party/abseil-cpp"
+angle_abseil_cpp_dir = "//flutter/third_party/abseil-cpp"
 angle_glslang_dir = "//flutter/third_party/vulkan-deps/glslang/src"
 angle_googletest_dir = "//third_party/googletest/googletest/src"