Generate thumbnails in the browser process.

The feature is now behind --enable-in-browser-thumbnailing flag.

The in-browser thumbnailing works as follows:

- The scroll offset is sent from the renderer to the browser.
- The thumbnail is taken when the page info is sent from the renderer process.
- Since thumbnails are generated in the browser, we can avoid the generation when unnecessary (ex. off-the-record mode, or New Tab Page).
- The quality of thumbnails is as good as before, as the patch doesn't change timing heuristics.
- The drawback is that we cannot take thumbnails from background tabs.

New functions are added to ThumbnailGenerator for clipping thumbnails, with tests.

BUG=65936
TEST=add unit tests for thumbnail_generator. confirmed that the thumbnails are updated with and without --enable-in-browser-thumbnailing

Review URL: http://codereview.chromium.org/6246007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@72115 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 5 insertions, 0 deletions

diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 1a86bcd..f781baf 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -497,6 +497,11 @@ const char kEnableFastback[]                = "enable-fastback";
 // testing, for example page cycler and layout tests.  See bug 1157243.
 const char kEnableFileCookies[]             = "enable-file-cookies";
 
+// Enable the in-browser thumbnailing, which is more efficient than the
+// in-renderer thumbnailing, as we can use more information to determine
+// if we need to update thumbnails.
+const char kEnableInBrowserThumbnailing[]   = "enable-in-browser-thumbnailing";
+
 // Enable IPv6 support, even if probes suggest that it may not be fully
 // supported.  Some probes may require internet connections, and this flag will
 // allow support independent of application testing.