Fix uninitialized member from PageIndicator::PageIndicator().

Per msan report, the line in question reads:
  if (timer_id == fade_out_timer_id_) {
so initialize fade_out_timer_id_ in the constructor.

BUG=411177

Review URL: https://codereview.chromium.org/557493002

Cr-Commit-Position: refs/heads/master@{#294055}

pdf/page_indicator.cc

@@ -15,6 +15,7 @@ namespace chrome_pdf {
 
 PageIndicator::PageIndicator()
     : current_page_(0),
+      fade_out_timer_id_(0),
       splash_timeout_(kPageIndicatorSplashTimeoutMs),
       fade_timeout_(kPageIndicatorScrollFadeTimeoutMs),
       always_visible_(false) {