dpkg-shlibdeps: Handle missing shlibs:Depends= line

On older versions of dpkg-shlibdeps, the line was omitted rather than
being "shlibs:Depends=\n".

BUG=1041360
TBR=thestig
CC=​​​wuwang
NOTRY=true

Change-Id: I66263774564ac3df9c083d3a79a3de54bd1bfea6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2004955
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732516}

chrome/installer/linux/debian/calculate_package_deps.py

@@ -68,7 +68,11 @@ if exit_code != 0:
   print 'stderr was ' + stderr
   sys.exit(1)
 
-deps_str = re.sub(r'.*shlibs\:Depends\=', '', stdout.replace('\n', ''))
+SHLIBS_DEPENDS_PREFIX = 'shlibs:Depends='
+deps_str = ''
+for line in stdout.split('\n'):
+  if line.startswith(SHLIBS_DEPENDS_PREFIX):
+    deps_str = line[len(SHLIBS_DEPENDS_PREFIX):]
 deps = deps_str.split(', ')
 interval_sets = []
 if deps_str != '':