Update linux-bfcache-debug / content_browsertests filters.

The bot is not running in debug mode, causing one new test to fail:
SecurityExploitBrowsertest.InvalidBeginNavigationInitor.

The bug is easy to fix, which I already have a fix for.

TBR: lowell@chromium.org

Bug: None.
Change-Id: Icd4e2b52829db155e5644e207f10ccae2932fc9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1816472Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Lowell Manners <lowell@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698486}

testing/buildbot/filters/bfcache.content_browsertests.filter

@@ -81,3 +81,6 @@
 # https://crbug.com/1004786
 -SitePerProcessBrowserTest.TestChildProcessImportance
 
+# In debug mode. The FrameHostInterceptor fails, because it doesn't take into
+# account pages in the BackForwardCache.
+-SecurityExploitBrowserTest.InvalidBeginNavigationInitiator