Added Abseil as a third-party dependency (Take 2)

Another attempt at https://dawn-review.googlesource.com/c/dawn/+/63780
Depends on https://skia-review.googlesource.com/c/skia/+/451638

This version of the CL fixes the Cmake and Skia compatibility issues
from the previous CL.

Bug: dawn:563
Change-Id: I41a7ee0aef1924df08a9be41e94effb7cd0025df
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/64600
Commit-Queue: Brandon Jones <bajones@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
diff --git a/.gitignore b/.gitignore
index 1b20edd..38f94ca 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,6 +7,7 @@
 /build
 /buildtools
 /testing
+/third_party/abseil-cpp/
 /third_party/angle
 /third_party/clang-format
 /third_party/glfw