A crash handler needs a way to clean up after itself it it calls
CrashReportDatabase::PrepareCrashReport() to begin writing a new crash
report, but then encounters an error that renders the crash report
unusable. The new ErrorWritingCrashReport() method allows it to
communicate to the database that a previously-prepared crash report
should be removed without ever being promoted to a completed report
pending upload.
TEST=client_test CrashReportDatabaseTest.ErrorWritingCrashReport
R=rsesek@chromium.org
Review URL: https://codereview.chromium.org/904533002