[WPT sync] Double the default history window for landed commits

This CL only increases the default history window size for *landed
commits* to 10000, which is about a month's worth of commits. The
history window size for GitHub PRs and Gerrit CLs are not changed.

Landed commits are scanned locally (using git), which is pretty fast
(takes few minutes), so doubling the history window won't affect total
run time too much.

Bug: 752214
Change-Id: Id2f498aa1d8a8adb8369fe7553c73662e5b02343
Reviewed-on: https://chromium-review.googlesource.com/1085556
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565302}

third_party/blink/tools/blinkpy/w3c/chromium_exportable_commits.py

@@ -7,7 +7,7 @@ from blinkpy.w3c.chromium_finder import absolute_chromium_dir
 from blinkpy.w3c.common import CHROMIUM_WPT_DIR
 
 
-DEFAULT_COMMIT_HISTORY_WINDOW = 5000
+DEFAULT_COMMIT_HISTORY_WINDOW = 10000
 
 
 def exportable_commits_over_last_n_commits(