[Messages][SaveCard] Reset metric recording related variables.

ChromeAutofillClient is instantiated once per WebContents; it's possible
to trigger save card prompts multiple times.

Some variables, like is_dialog_show_, if set true, should be reset
after promotion is completed; otherwise, some metrics are wrongly
recorded.

Bug: 1190531, 1337107
Change-Id: I962078bca3975f6dfa02f64306a73bd717daf0b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3710952
Reviewed-by: Elizabeth Popova <lizapopova@google.com>
Commit-Queue: Lijin Shen <lazzzis@google.com>
Cr-Commit-Position: refs/heads/main@{#1015544}