misc files R-U: Change ScopedTempDir::path() to GetPath()

path() is being deprecated, GetPath() has better checking against wrong use.

For more context, see https://codereview.chromium.org/2275553005/.

BUG=640599
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.win:win10_chromium_x64_rel_ng

Review-Url: https://codereview.chromium.org/2317123003
Cr-Original-Commit-Position: refs/heads/master@{#421483}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a98198273f744d4e045e759734bbaa69962fa94d