Add .gdb_history to .gitignore.

R=machenbach@chromium.org
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/1486083002

Cr-Commit-Position: refs/heads/master@{#32458}

.gitignore

@@ -25,6 +25,7 @@
 .cproject
 .d8_history
 .gclient_entries
+.gdb_history
 .landmines
 .project
 .pydevproject