Fix include paths for pdfium_unittests under chromium checkout.

Here, testing/gtest and testing/some-pdfium-stuff will be different
directories, since we will use the gtest at top-level.  For standalone,
they happen to be the same.

TBR=bo_xu@foxitsoftware.com

Review URL: https://codereview.chromium.org/852493002
2 files changed