win: Fix module timestamp test

win: Fix module timestamp test

This test was added in https://codereview.chromium.org/1052813002. It
was previously checking the timestamp from in-memory module traversal
vs. the disk mtime. This is flaky (of course) because it depends on
the linker writing the header and closing the file during the same time
quantum. So the bots occasionally failed with:

[ RUN      ] ProcessInfo.Self
e:\b\build\slave\chromium_win_dbg\build\crashpad\util\win\process_info_test.cc(86): error: Value of: GetTimestampForModule(GetModuleHandleW(nullptr))
  Actual: 1431650338
Expected: modules[0].timestamp
Which is: 1431650337

Instead, use imagehlp to pull the timestamp out of the header so that
it matches the header value that will be the in-memory timestamp.

R=cpu@chromium.org
TBR=mark@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/8171e6f76591536e0670c76f42a31f3ccd37a90e

[cpu_(ooo_6.6-7.5), 2015-05-15 01:42:52]

lgtm

[scottmg, 2015-05-15 01:45:33]

Committed patchset #1 (id:1) manually as
8171e6f76591536e0670c76f42a31f3ccd37a90e (presubmit successful).