Avoid negative text_pixels LCDTextMetricsReporter

- Change the type to int64_t which is the same as the parameter
  of base::ScaledLinearHistogram::AddScaledCount().
- Use base::checked_cast when casting from double to int64_t.

Bug: 1423504
Change-Id: Iba85ca4fbf49e54c8cdc32c4001a0fd9f1bb4f5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4343035
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1117923}