Click to Open PDF: Enable on trunk

Enabling on trunk in preparation for the 100% Stable rollout. 1% Stable
experiment has concluded with positive metrics consistent with the Beta
experiment.

Bug: 799085, 737787
Change-Id: I656f55dd100ca97973e417c11bbb494f4e3c4fa9
Reviewed-on: https://chromium-review.googlesource.com/1055929Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558438}

chrome/common/chrome_features.cc

@@ -160,7 +160,7 @@ const base::Feature kClearOldBrowsingData{"ClearOldBrowsingData",
 #endif
 
 const base::Feature kClickToOpenPDFPlaceholder{
-    "ClickToOpenPDFPlaceholder", base::FEATURE_DISABLED_BY_DEFAULT};
+    "ClickToOpenPDFPlaceholder", base::FEATURE_ENABLED_BY_DEFAULT};
 
 #if defined(OS_MACOSX)
 const base::Feature kContentFullscreen{"ContentFullscreen",