Looking into replacing paper-header-panel with something else. All of Settings, History, Downloads achieve the same drop shadow effect without paper-header-panel.

Having said that, currently in  MD extensions the drop shadow is always visible, instead of showing up only when the page is scrolled down, which is not desired.

Candidate fix is at https://chromium-review.googlesource.com/c/chromium/src/+/679275.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5b5076342f57e659d25d45ecc1133901714aae9d

commit 5b5076342f57e659d25d45ecc1133901714aae9d
Author: dpapad <dpapad@chromium.org>
Date: Wed Sep 27 04:23:20 2017

MD Extensions: Remove paper-header-panel, fix drop shadow effect.

 - Extracted IntersectionObserver logic from Settings to a new reusable Behavior.
 - Removed paper-header-panel usage from MD Extensions
 - Using the new Behavior from MD Extensions.
 - Moved drop shadow effect styling in settings_vars_css.html and using it from
   all 4 pages (extensions, settings, downloads, history).

Bonus: paper-header-panel can be completely removed from Chromium after this CL.

Bug:  751758 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I5679c3297b1f8c85d68a1d7538c3055dcc62c5e9
Reviewed-on: https://chromium-review.googlesource.com/679275
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504580}
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/chrome/browser/resources/md_downloads/manager.html
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/chrome/browser/resources/md_extensions/compiled_resources2.gyp
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/chrome/browser/resources/md_extensions/manager.html
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/chrome/browser/resources/md_extensions/manager.js
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/chrome/browser/resources/md_extensions/toolbar.html
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/chrome/browser/resources/md_history/app.html
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/chrome/browser/resources/settings/settings_ui/compiled_resources2.gyp
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/chrome/browser/resources/settings/settings_ui/settings_ui.html
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/chrome/browser/resources/settings/settings_ui/settings_ui.js
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/ui/webui/resources/cr_elements/compiled_resources2.gyp
[add] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/ui/webui/resources/cr_elements/cr_container_shadow_behavior.html
[add] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/ui/webui/resources/cr_elements/cr_container_shadow_behavior.js
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/ui/webui/resources/cr_elements/shared_style_css.html
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/ui/webui/resources/cr_elements/shared_vars_css.html
[modify] https://crrev.com/5b5076342f57e659d25d45ecc1133901714aae9d/ui/webui/resources/cr_elements_resources.grdp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/02b7e5fa7bc98300eebfb88192e66d1f1d2d0ba7

commit 02b7e5fa7bc98300eebfb88192e66d1f1d2d0ba7
Author: dpapad <dpapad@chromium.org>
Date: Wed Sep 27 20:42:08 2017

WebUI cleanup: Remove unused paper-header-panel Polymer element.

Bug:  751758 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I0552b674bf1ab78e2ab942a79d7d98bf0da181d3
Reviewed-on: https://chromium-review.googlesource.com/679196
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504764}
[modify] https://crrev.com/02b7e5fa7bc98300eebfb88192e66d1f1d2d0ba7/third_party/polymer/v1_0/bower.json
[delete] https://crrev.com/ed567ae23c1096c70f000dafc0baad2ea5ac0723/third_party/polymer/v1_0/components-chromium/paper-header-panel/bower.json
[delete] https://crrev.com/ed567ae23c1096c70f000dafc0baad2ea5ac0723/third_party/polymer/v1_0/components-chromium/paper-header-panel/compiled_resources2.gyp
[delete] https://crrev.com/ed567ae23c1096c70f000dafc0baad2ea5ac0723/third_party/polymer/v1_0/components-chromium/paper-header-panel/paper-header-panel-extracted.js
[delete] https://crrev.com/ed567ae23c1096c70f000dafc0baad2ea5ac0723/third_party/polymer/v1_0/components-chromium/paper-header-panel/paper-header-panel.html
[modify] https://crrev.com/02b7e5fa7bc98300eebfb88192e66d1f1d2d0ba7/third_party/polymer/v1_0/components_summary.txt
[modify] https://crrev.com/02b7e5fa7bc98300eebfb88192e66d1f1d2d0ba7/ui/webui/resources/polymer_resources.grdp