Fix failed to write thumbnail png issue

In https://codereview.chromium.org/2594243002/, filepath of png output is
wrapped to GetTestDataFile(), which will use base::MakeAbsoluteFilePath() to
get absolute path. However, since the output file doesn't exist (yet), this
function will return an empty FilePath instead and makes write file failed.

BUG=701232
TEST=Run Thumbnail/VideoDecodeAcceleratorParamTest.TestSimpleDecode/0 of VDA
unittest and make the test matches no md5sum. The test will generate
bad_thumbnails.png successfully.
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2753773002
Cr-Original-Commit-Position: refs/heads/master@{#458706}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 58af2e5f92c3c5706d7aba46169001c618684169
1 file changed