Fix frame_editing_unittest.cc
Fix frame_editing_unittest.cc

The test fails since it's assuming out/testfile.yuv exists when running the test. Just opening the file at a later time than the SetUp function seems to break the test so that's not a viable solution. This CL uses a simple workaround that simply truncates the file before opening it, which works.

BUG=none
TEST=tools_unittests in Debug+Release on Mac, Win and Linux + memcheck, tsan, asan.

Review URL: https://webrtc-codereview.appspot.com/1067004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8126602e26f6f632d846aeb570c8fdfe9e67bfcf

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